As I understand it the car rules in JGTC are fairly lax except that you have to use the engine from a car currently in production hence:
Supras used 2JZ then 3S-GTE now 1UZ-FE
GT-R used RB26-DETT and now VQ35-DETT
The GT300 MR-S can use the celica motor for more go and I think turbocharging is allowed in GT500 hence TT VQ35.
Yeahthis is my taste of Celica ....it's like a Dream come true...(new AE86? )
Sadly , it's only the looks ....2003 JGTC spec for GT300 has been change .almost close to GT500 ...with Flat chansis , allow maximun cablebility of suspension And Performance part in use ...