RE: .051 vs .049
This approach goes a long long way back-we've touched on the TD 049/051 (there was also a Medallion 051 produced for a very short time-essentially in response to NFFS pressure for a Nostalgia legal engine) Holland did it with the Hornets and so did Atwood.
In the larger sizes you had the K&B 19/23, Ohlson ditto, Fox 19 and the (very rare) .201, and Forster 29/31 plus as has been mentioned K&B 29/32...
As always the intent has been to allow the same aircraft to compete in two different categories simply by changing the engine. There have been a few odd ball 'mutants' done over the years as well-the Chamberlain .42 which was several different models of OS 40 combined to produce a class D F/F motor. The other side of the coin-overboring an existing motor to compete in the next class up-a few of the Brits experimented with 2.6cc diesels (such as an overbored Eta 15) in class B team race. Of course the opposition was .29 glows, so it was a totoise and hare approach.....
If you want to get creative you can interchange bits on the K&B 20 and 28 Sporters to produce two hybrid capacities as well.....(but why would you want to.........)
'ffkiwi'