Given the Nelsons were 1500 tons under the treaty limits as built, I think there's some scope to increase horsepower while keeping the gun and armor characteristics and without undue structural modification to the design. Something like an 80,000 shp plant is probably doable; anything more, though, requires more shafts and a corresponding deep redesign.
Sadly, this only gets us to about 26 knots or so. So this really does need either legal shenanigans or a major redesign and reduction of either guns or armor.
Something I came across looking at CVA01 which might also appy to the Nelsons is the 'packaging' of propulsion machinery. The 53,000t CVA01 had 3 shafts, but to go to 4 shafts required 58,000t but provided no tactical advantage for the cost and complexity.
Would something similar apply to the Nelsons? To harness a significant increase in power would require a 3rd shaft which can't be accommodated in 1500t?