the only option is getting the gt4, the standard st162 wasn't built to handle the power out put of a turbo. Best to get a GT4 out of a 90s model celica.
have a look at the thred "st162 turbo upgrade". i got my gt4 out of a gen 5 celica and am having a few problems. also (not mentioned in that thred) my wiring harness had to be pulled apart and put back together with matching plugs, as the plugs on gen 5 are not the same as on the gen 4.