If you want to get to the magic 180degree number without butchering the liner, make a liner shim out of .005" brass so you can raise it. You can use the liner as a template and score the shim stock until it slices through. Cuticle scissors can also be used for cutting them out.
The guys at the NASS would be a lot of help with this size engine as well as the guys at those German RC forums. I'm just guessing that a 6x6 prop would be pretty close, but the guys who've actually run this engine for RC speed could save you time and money.