J15 are available, good motor more reliable than an A15 and used in forklifts. Some come up from time to time for $50 or free on ozdat.

here is an overpriced one but have a look at the exhaust manifold its a work of art. The engine is based on the MG but much better metallurgy and JP quality. Just needs a single SU on that manifold using an adaptor, a mild cam and it will be a good little performer.