BMC in OTL had around two engine development paths to choose from that can essentially be dubbed as the Nissan and Volkswagen routes, each with their pluses and minuses.
Out of those two engine paths, which one would BMC have been better off pursuing assuming the company had the same level of competency as Nissan and Volkswagen respectively?
1)- Nissan (aka Evolutionary Engine Design) as embodied in ideal developments to the A-Series (also A-Plus/A-OHC) and B/O/M/T-Series (plus L/G-Series diesels and Project Storm modular 4/5/6-cylinder diesels) plus their successors, by drawing inspiration from the Nissan A/E, MA, CG/CR for A-Series descendants and CA/SR engines (with the CA being an indirect descendant of the B-Series as it was an up-scaling of the Nissan E engine).
Interestingly Nissan had some pre-existing links with Austin up to the 1950s and basically developed new engines derived from improvements to the A-Series and B-Series engines, with a number of Nissan engines sharing some distant relation and other elements to the Austin units to the point where certain parts on the latter can reputedly be switched with Nissan parts with little to no issues.
Ideally come the mid-80s to early-90s in this ATL BMC scenario the A-Series (now an OHC from early-70s) would be replaced by a British built analogue of the Nissan CG/CR in 1000-1600cc forms (capable of spawning 3-cylinder, diesel and turbocharged variants), while the ATL M/T/L-Series would be replaced by an alternate petrol and diesel version of the modular Project Storm engine family in 4/5/6/8-cylinder engines.
In the case of 6-cylinder and V8 engines, it has been demonstrated that 6-cylinder versions of the 1200cc A40 / B-Series engines were developed. With the A40-derived inline-6 taking on the C-Series moniker prior to being abandoned and the B-Series forming the basis of the Blue Streak engine. While Tadek Marek before leaving Austin to move to Aston Martin developed an A40-derived V8 engine that did not reach production.
2)- Volkswagen (aka Clean Sheet Engine Design) as embodied in the 9X and E/S-Series plus their successors via the Volkswagen EA111/EA211 and EA827/EA113 engines
In the case of the E-Series in spite of preceding the Volkswagen EA827 the former has a number of similarities to the latter such as both being undersquare / long-stroke engines despite the E-Series being limited due to the bore centres being 6mm shorter compared to the EA827 as well as having siamised bores and being a tall engine (the latter of which notoriously ruined Harris Mann’s original sketch for the Austin Allegro in OTL, an issue that could have been mitigated with an end-on gearbox instead of BMC’s in-sump layout).
However despite the E-Series being originally conceived at BMC to replace the A/B-Series and C-Series engines at once, the OTL Volkswagen EA827 demonstrates why in reality a 60 hp 1.3-litre EA827 OHC let alone an E-Series of the same displacement would NOT have been a significant improvement over the existing 1.3 A-Series OHV that already produced similar power or even the 63+ hp 1275cc A-Plus and unbuilt 84 hp 1275cc A-OHC engines (the E-Series was designed to go as low as 1160-1300cc with the Volkswagen EA827 itself initially being conceived as a 1200cc engine).
3-cylinder versions of the E-Series and EA827 were also developed with the latter actually being produced in both petrol and diesel forms (with earlier versions considered for replacing the old 2-strokes at Wartburg - petrol and Trabant - diesel prior to the end of the Cold War), while the former formed the basis for the loosely related ECV3 engine. However it seems both Issigonis (designer of the E-Series) and Volkswagen felt both were inadequate for engines at the lower of end of the range, hence explaining the rationale for both the 9X and EA111 engines (the latter being a downscaled EA827) that also seem to share some similarities (with the latter giving some insight as to how a properly developed 9X engine could have evolved had it reached production).
Curiously it seems the EA827 also formed the basis for V8 and even V10 engines, the former in the Audi V8 (which spawned 90-degree V6s) and the latter derived from a pair of 5-cylinder EA827 Audi engines as an alternative to the V8 in the Porsche 928 prior to the V10 eventually finding its way to Lamborghini.
Notes:
The OTL Rover K-Series does not figure in this scenario since despite being developed at BL (which does not exist in this scenario) as a replacement for the A-Series, it was largely a design that owed much to Triumph rather than BMC. Additionally it could not be fitted into an original Mini as a 4-cylinder and BMW during development of the BMW MINI even dismissed the K-Series for not being compact enough against the Tritec engine, which notwithstanding BMW’s OTL ill-intentions towards Rover is worth mentioning in light of the fact that the Nissan CG/CR engine in the Micra K11/K12 are in fact compact enough to easily fit into the space of an original Mini (and being a fairly popular engine swap).
Am also NOT focusing on other outside engine options explored at BMC such as the Lancia V4 inspired 18-degree V4/V6 engine family (dismissed due to tooling costs as well as inability to be mounted transversely), despite Volkswagen later developing the similar VR6 engine family (spawning VR5 along with W8/W12 engines) or hypothetical A/B-Series analogues derived from the Morris-developed BMC C-Series (entailing earlier PODs).