The rust problem is consistent with the use of a muffler. It can happen virtually overnight.
Corrosive by-products of combustion in the exhaust residue condense inside the engine and attack steel parts. I have seen this happen to several engines fitted with large, expansion mufflers. It does not seem to affect PAWs with their smaller muffler volume.
It is more likely connected to the ether content, rather than the inclusion of additives such as MEK.
The solution is to flood the engine with a dwf such as WD40 after a day's flying, to close the exhaust port and to ensure that the model is not hung nose down.

David Owen