I'd be interested to get some pointers on the UL situation for an aircraft like the Yak52. In practice, it's likely to be irrelevant for our group as the timescale suggested (2020?) would put us close to a viability decision anyway, based on airframe hours.
Other than strapping a PT6
or that 500hp German diesel V12 on the front (oh yes!
), either of which you could probably get away with in FAA land, would UL be suitable for a Vedeneyev radial? A better question is perhaps, would the Campaign Against Aviation allow continued operation on 91UL if it is?