You're gonna look for a four-bolt T3 flange turbo, if I recall correctly. I don't know which turbos are bolt on, most everyone I've talked with made a hybrid out of theirs.
If you can get them, you can upgrade the turbo with stock Nissan parts. It's not the way to go for extreme modifications, but it is good for basic upgrades. You can put the RB25's turbo in, which is what I'm working on doing. It's a direct fit. Another upgrade that was popular with the drifters is to put the turbo from a VG30DET (single turbo) in. It mounts up with the stock manifold, you do have to swap it's exit pipe with yours though.
I'm sure someone will follow up with good bolt on turbo setups. The above is good it you're on a limited budget (like me) and can get ahold of the parts.
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1990 Skyline GTE/S-t 4 Door Updated 6 Mar
Swapped out RB20E with RB20DET
Trans converted from Auto to 5-Speed...
Now with SR20DET 5-Speed
Installed Type M 5-lugs and brakes, R32 GTR rims and 2-Dr rear end (Tail lights & bumper)
1994 Isuzu AMU
US Spec Amigo with JDM Isuzu MU (Mysterious Utility, as it's called in Japan) nurf bars and hatchback style hardtop.