Engine sizes vary from 80cc 2/stroke to 200cc 4 stroke depending on which class you wanna race in! You didnt go ant look at those links ppl put up did you garath, hehe. I`d buy one ready made as you`ll need engineering experience for about 100% of the mission!
One of our friends (stuart) is making a 125 4 stroke and i`m sure he was upto about £1500 with it only half built! Does look nice though and has planty of trick bits on it!
Do you even race these in Ireland??

I`d like to think so wouldnt you MrCarver!!??