swapping in any 4 cylinder engine into the z31 would be shooting yourself in the foot with a rocket launcher.
for high performance there is nothing more than a vg30et needed... rebuild the internals with z32 rods and forged pistons, and do some headwork and the engine is good to 600+hp, now that's with supporting mods of course...
and as for the 'performance is not that great' you've obviously never ridden in one that's got <$1000 worth of mods in it (exhaust intake and boost controller)... on a cool day with good high octane gas it can rival the acceleration of a 350z...
if you absolutely must swap an engine, look into the rb series... the rb20det would be the easiest seeing as they came stock in the 200zr in japan, would only require a front clip or some custom mounting and so on.... next up is the rb25det or 26dett, both will require quite and extensive amount of fabrication and custom work and isn't really worth it unless you're going for the show car or unique(and extremely expensive) look
if you're that much into 4 bangers, i'd stay with the lighter 240sx/sylvia chassis.
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ZedEx Crew Member #6
1988 300zx turbo 5spd. 3" mandrel exhaust, filter, afco rad, e-fan, poly engine mounts, mbc at 8.5 psi, turboxs rfl-h bov, gutted plenum, etc.
blown turbo, under construction.. gt35 coming.