Early Celicas were delivered to other European countries, so that might be your best bet for sourcing parts, otherwise if there is a part that you are desparate for, the exchange rate between Euro/Au$ and Euro/Yen is pretty good for you, so ask around you could probably get the parts shipped to you for a reasonable cost. Why not get the celica bits you want chucked into a MR2 3S-GTE half cut and import everything from Japan.
Simple mods for your car......where do you start.
Me I would make sure that the body work is rite, then do wheels and suspension. Then you will have a nice car to run around in.
The first performance mods I would do is improve the brakes. There is heaps of options available to you there, have a hunt through this site.
Then whack on a good exhaust and a couple of webbers and you'll be driving with a big grin