Are you kidding? There are heaps of people on ToyMods with RA60's. Go back and have another look!
The most common conversion for the RA6x Celica these days is the 1G-GTE twin-turbo 2 litre six. It goes in relatively easily and with up to 200hp on tap it really wakes up the old Celica. Compare this to your current 21R-C which has less than 100hp, and you can see it's a pretty good upgrade.
The 4A-GZE is a small engine which is suited to small cars, like the AE86 Sprinter for example. The Celica has more weight to haul around, so you want something a bit bigger. And since you have more engine bay space to play with, why not chuck something decent in there?
BTW $8.5k isn't a lot of money for a radical engine conversion. If this figure scares you, you should either consider doing the job yourself or give up on the idea completely. Engine conversions are NOT cheap.