Thunder Tiger .54. OS.52, Magnum.52 (or is it a Magnum.53?) Saito .50 or .56. are all suitable.

Bear in mind that those with an OS style cylinder head design (the 1st 3 mentioned) can sometimes be difficult to start if mounted at 180 degrees inverted. (Head profile allows flooding of the glowplug.) Mounting the engine so the head is at 190 degrees solves the problem.

The Saito engines don't have that problem.

But I guess i'ts whatever you can get a hold of at the most reasonable price. They're all good engines with power output about the same.