Ok, when I first saw this I noticed Bubba saying an OS 91 or Saito 100 is ideal, but he thought you had the 60 size H-9 Mustang.

For the PTS, go with an OS 70 Surpass (That would be my choice) or a Saito 72

The Saito 82 is WAY more than it needs, and this plane is a little nose heavy even with a 2-stroke, so I would really avoid going too big with the engine.

(Note: H-9 recommends a 4-stroke up to an 82 simply because they MAKE an 82. I've got a PTS, and a 70 is as big as I would ever go with a 4-stroke)