The PT-19 appears to be a very versatile plane. I fly with a group that powers theirs with Zenoah G-26 gas motors, and they all fly very very well. Your 28 CC motor may be just the thing if you want real horsepower in it...and relatively inexpensive fuel too. The OS91 seems to be way too small to get any good flying out of this plane, and the 108 is only just a little better than the 91. Good luck with yours.
Jim