What would happen to the economy?
Well, there are three main strands of divergences which have to be considered, including their interplay:
1) The Bolsheviks keep on fighting and thus need the war-geared industry to keep on working
2) A much reduced civil war and possibly even reparations as dividends from 1919ff.
3) Trotsky favouring different economic policies than Lenin.
(3) has been discussed often: Trotsky favoured fast industrialisation and might never have even started the NEP. When food production and distribution would turn out to be a major problem, Trotsky is expected to push collectivisation earlier.
So far, so boring - but we have to combine this with (1) and (2) now.
(1) probably means reactivating the wartime dirigist administration inherited from the tsarist empire and sovietising it, at least for the remainder of the war. On the other hand, once the institution is established and running under communist control, then I don't see why one would create something like Gosplan from whole cloth when you already have an institution capable of doing pretty much the same thing. So, as far as industrial management is concerned, there's probably comparatively more personal continuity, which is not a bad thing. Then, there is the question of the tsarist debt and how to keep up the economic war effort. Trotsky was not exactly an able negotiator, so don't expect the maximum from him here (the maximum probably being international recognition and a serious haircut on the inherited debt plus new loans or even free materiel if only the Russians keep on fighting), but with industry tanking either way (it was collapsing all throughout 1917 already), Trotsky's war effort needs some foreign help, which has a lot of interesting implications and might mean that the nascent Soviet Union is not quite as economically isolated as it was IOTL.
(2) I don't think Trotsky would charge reparations from a socialist Germany, but if Germany develops roughly along OTL lines, the Soviets might want and get their share of reparations. Combined with much less civil war and more continuity in industrial management, this might mean that the food crisis which IOTL caused Lenin to turn to NEP and which might turn Trotsky to early collectivisation does not yet hit in 1920 or 1921.
So it is not exactly a given that Trotsky at the helm means early collectivisation, full stop, end of discussion. In this scenario, things might go differently.
The problem of very probable industrial mismanagement and a dawning scissors crisis can still come up, but when? That's not set in stone. Also, if there is more extrenal trade, then even a mismanaged industry might mean peasants who now have land could possibly sell and save for an American tractor... if there are no decent Russian ones around.
Trotsky's emphasis on fast industrialisation might mean that if the Soviet Union gets reparations, they go straight into railroad repair and industrial build-up. A five year plan already for reconstruction?