If you want to stick to the A series, A14 or A15, have a talk to BigE, His business is building A series engines for Speedway, but has also built street motors for members. Try sending a PM or call ATE engines on 0412864741 or email ateracing@hotmail.com. You can also try Stewart Wilkins at SWR Rallying on 02-4577-2400 or www.swmotorsport.com.au, or Jeff Taylor at JTS.
If you want to try an L series or a CA18, again call SWR. They have a large workshop near Windsor & Stewart's been rallying cars for a couple of decades.