I think you will find that, for any factory built aircraft, the maintainance manual hours MUST be adhered to, unless the local athourity (casa or RAAus) grants an excemption. This would mean that the 300hr TBO on the Rotax two strokes is mandotory, as is the previously mentioned 1750hr TBO for the 912. This time limit apply's to private use also.
As for EXPERIMENTAL aircraft, you can go byond this limit, BUT, the engine then also becomes experimental. This may limit the airspace in which you fly (legaly!),
When it comes to auto conversions, these too are experimental engines, and as such do not have a TBO. These enginess cannot be fitted to factory built aircraft, for obvious reasons.
There is a list (I think!) of approved engines in the RAAus system. These engines can be used over built up areas. If the engine in your aircraft is not on this list, it would be wise to check out it's limitations.
There is, I think, a difference between RAAus experimental and GA experimental, when it comes to engine use over built up area's. Perhaps someone on this forum can elaborate.
What some schools do with their two stroke engines is replace them at TBO with a new engine, and sell off the old one for use in experimental (or kit built) aircraft.
Thats all for now!
Gerry....