Easiest things to do:
- lowered and stiffened suspension (springs and shocks)
- mags (from old 4 stud Toyotas, Nissans, Mitsubishis)
- free flowing exhaust (aftermarket manifold, 2" pipe)
Slightly harder:
- replace 20R with 22R (carburettor) or 22R-E (EFI)
- use 20R head on 22R block (raises compression ratio)
- change diff ratio (changing 3.727 to 3.909 gives snappier acceleration but lower top speed)
- change solid rear axle to IRS (with disc brakes) from MA61 Supra ('82-'85) or RA65 Celica GT-S (83-85).
Whole hog:
- 3S-GTE turbo engine or 1UZ Lexus V8 engine (and yes, both have been done)
More details at
http://members.iinet.net.au/~stepho/celfaq.txt
It's more for the 75-77 model but much of it also holds true for the RA42/43 (my '77 liftback now has many RA40 parts on it).
More old Celica owners at
http://autos.groups.yahoo.com/group/classic-celica/
- Stepho
http://members.iinet.net.au/~stepho/celprod.htm