I have a thunder tiger .21 FSR ABC marine engine and would like to run a tuned pipe on it but I can't seem to find one. There is not very much info out there about this engine! Does anyone know of a header that would fit? I know Macs can make one but I dont really want to pay $45-50.00 for a sport engine header.</p>

I cant believe that nobody has come across one of these engines

Thunder Tiger doesn't make a tuned pipe or manifold for this engine, so you'll have to go with one of the after-market makers. Tuned pipe systems are not cheap, as you've found.