What other engine 4 cylinder would you fit to a E30

[jkarran]

Hyabusa turbo. Might find rallying it kills the gearbox/clutch though?

Redtop, NA Cossie, Duratec, S2000 are probably your best NA options for a 4-pot rally car. No problem getting RWD bits for any of those.

As someone else has mentioned, the MSA rules on drivetrain swaps are changing in the next year or two (sorry, not sure exactly when). Look into it carefully so you don't end up with an expensive trackday car.

jk