Read stuff in these forums first, and the old toymods forums.
Pistons are easy to get, if you pay heaps of money cutom ordered. Or you can use standard 3T pistons, max size is 86mm I think. THe exhaust valve relief needs machining out. There is a cast Repco piston avaliavle in 89mm for a 3T, but it has full skirts that need to be ground off (I have seen this done, not very nice).
Talk to Sean Malony, he will help. But do research first or you will waste his time. Standard single valve springs are ment to be good for 8000rpm, and are ment to be better than the double springs in the 2TGEU.