Quickboost has some nice ones, but you'll need to add your own pushrods and ignition harness:
IIRC Eduard has a set of B-17 engine details.
Ummmm, I assume you're talking 1/48?
Found this over on Airliners.net, hopefully it'll be of use:
This is the search I used:
Ray, while I can't say with 100% certainty that this is the case, the only XP-47J conversion I can think of was the one Bill Koster did for Monogram's 1/48 Razorback.
I am willing to be proven wrong, though.