You can get a brand new Magnum 4 stroke for only a little more than the used ones your looking at. The Magnum .91 is 189.99 at HobbyPeople.net right now. Magnum .52 149.99 and a Magnum .70 is 169.99. Everyone who has a Magnum .91 that I've read on here loves it. I don't have one myself, but I do have a 1.20 Magnum (also a great 4 stroke) in the TF P47-D. Magnum is a great "bang for the buck" engine. Your going to hear people in this thread tell you they are O.S. copies, which they are, but they are also good reliable engines, don't let it sway you if you choose to look at them. You can also look at the Thunder Tiger 4 strokes. The Thunder Tiger 54s is 179.99, but the Thunder Tiger .75s is 169.99 at ThunderTiger4U.com. Thunder Tiger also has a .91 4 stroke but it's pretty heavy (heavier than other .91's), on the flip side it's also a powerful .91 4 stroke, I think it's 194.99. The only other 4 stroke I have is in a GP Corsair, Saito .82, it's a good engine, but they are also known to vibrate a lot. I got it early this year, well before Saito hiked their prices up, which is why I bought the Magnum 1.20 over another Saito. I probably won't be buying anymore overpriced 4 strokes as long as Magnum and Thunder Tiger keep making good 4 strokes.

Edit: I mention all of these engines because in your original post you advised you were trying to keep the cost down. No need to buy a used engine when you don't know it's history if you can get a brand new one for considerably less than a Saito, O.S. or YS.