There are aftermarket firms that make resin props. Don't know if there is one specifically for a Boston, but I suspect that was a popular engine, so there may be some out there. Of course, you could also try to get an exact replacement from Airfix, depending on HOW old the kit is. With the latter you contact Airfix directly. For the aftermarket ones, many of the larger online retailers like Squadron, Rollmodels, etc. have pretty good search engines to help you find things.