RE: Chosing a New Engine OS, Magnum, Thunder Tiger
I would ditto what Sport Pilot stated, trade the old engine in on a new Fox .45 or .50 engine. Fox offers a trade in policy where you can trade in a engine, any brand and condition and get a new Fox engine for 50% off the retail price. The new Fox carburetor is great and easy to adjust too.
Another option is to go with a MECOA engine such as the K&B's or something else they sell too.
But if those are your only choices then go with the TT Pro .46 engine.