the 2.4 stroker is a serious amount of work, and will be expensive. I think they use a Z series crank in the conversion, and crankcase needs to be machined for extra clearance for rod big ends. I'm not sure of all the internals they use.

L18's can be taken out to about 2.3 if you are very serious, but you'll have to spend alot of time to find a good block that will take this kind of oversize. They're getting pretty scarce these days. Probably need trick pistons as well in such a short block. But if you are serious about it, I can put you onto some people to source parts or build it.

Is this going to be a street car? A 2100 L20B would be a cheaper exercise, still with heaps of grunt - I'm sure he'd be happy with that for starters.