Robin - A year ago I spent a considerable amount of time evaluating the Aviastar .53 for John Brodak.
For initial tests, I ran it in per instructions and then machined a venturi, .30 as I recall, and installed an OS ratchet type needle valve (I prefer that style instead of the collet type). The stock muffler was nice and quiet but very heavy.
I then mounted it on the only plane I had available, a very heavy Polaris. The run was typical of a Schnuerle ported engine - too fast, lots of runaway.
As I do not have a lot of expereince with port timing but lots with head configurations, I opted for the latter. The stock configuration was a small hemispherical chamber with a large - approximately 1/8 inch squish band. Progressively narrowing the band while increasing the chamber radius eventually yielded some very nice running. The best prop was a 12-6 APC, which loaded the engine down but allowed a good wet 2-cycle run with sufficient leaning in maneuvers to keep the lines tight. When I started modifying the head, I also switched to a simple tongue muffler that I originally made for an ST 60.
The final head configuration was a .50 inch radius combustion chamber with a .04 wide squish band.
I ran muffler pressure and a non-uniflow metal tank and the runs were as consistant as I could want.
Around 5 ounces of Brodak 10/23 fuel was sufficient for the pattern.
I sent all the information back to the factory and I assume that they incorporated the changes.
I personally feel that it is a fine engine and reasonable priced.
Two things to consider:
I did no long-term evaluation so I cannot comment on the engine life.
The unit that I evaluated was a bit on the heavy side - around 2.5 ounces heavier than the ST .51.
Maybe they incorporated some weight reducing - I do not know.
Bob Z.