Mechanically, they run the G161z - tough, strong little engine capable of serious mods, serious power, and serious abuse. Simple to rebuild and keep running too. Not sure why Isuzu has a bad rep in america, down here in Aus their known as being unbreakable.
If the engine is totally screwed, you can bolt in a G180 or a G200 [1.8 and 2L] out of an Isuzu Pick Up or early Isuzu Trooper.
The 4ZC1, 4ZD1 and 4ZE1 [2L, 2.3 and 2.6] out of the later P'up, Impulse and Troopers also go in with very simple mods [swap engine mounts, minor trim to the firewall].
Gearbox's, any Isuzu RWD [4 or 5spd] from any of the above
listed cars bolts onto the back of the engine. Depending on which engine you run [lets assume its a G series] it should also bolt directly to the standard xmember. If however its a later hydraulic clutch box youll need to sort that bit out. Again, not hard.
Here are mine [one Coupe, one sedan]
[F3ARED] 76 Isuzu Gemini Coupe - forged G180z, EFI, 525hp Turbo, 6years and still building...
Daily 78 Isuzu Gemini Sedan, stock G161z, Rodeo EFI, 500hp roller, 157rwkw. Fun