RE: new demon to fly soon!!!
See if you can buy/borrow/trade/aquire a Thunder Tiger .40/.46 or something similar. Inexpensive and they run well. Fly it with that to start with.
Resulting mounting holes will be the same as will be used for the Jett.
The TT-39 engine has the OS .32 mounting bolt pattern. If you want to go that route, you may wish to make a u-shaped adapter plate that fits on top of the stock engine mount - perhaps from 1/8 AL or 1/8" ply --- mount the engine to that, and then bolt that entire plate into the front of the plane. Might serve as a break-way plate if things get a bit rought along the way.