Garrett t3 turbine and shaft with stock ht18 housing and compressor?
 

My stock turbine went out, and can't find any replacement turbines for the ht18s-2s. I already have the rebuild kit and I heard the Garret's t3 would fit. If I'm able to, do I need any supporting mods?

there are a few different T3 turbine wheels. the stage II is pretty close to the factory size, but it may need some milling to fit. Gpopshop can do the milling for $100ish

A P-trim turbine can be machined to fit the stock exhaust housing. It's been done on the larger 300+ WHP hybrids.

You can get a used turbine from the classifieds like the one below. I have no affiliation with the seller.
https://www.rx7club.com/2nd-generati...stuff-1095365/