O.S. 4-strokes are generally regarded as being equally as reliable as their 2-stroke engines. Saito 4-stroke engines are very popular because they are powerful and very, very durable while also being very light weight. 4-stroke engines from other manufacturers like Thunder Tiger and Magnum also enjoy a following and generally have good reputations.

A .91 Four Stroke is a good fit for a Sig Four Star .60. As previously stated, a .70 4-stroke would be a bit underpowered, it would be like putting a .46 2-stroke on it. It would pull the plane around and be flyable, but it wouldn't be as much fun for aerbatics and such. An .80 to 1.00 4-stroke is probably the ideal range.

You've had good experiences with O.S. Max engines, and if you buy the FS-91 II Surpass for $249.99 you won't be disappointed. It's a strong, reliable engine that will give you many years of great flying service.

A Magnum, Thunder Tiger, or Saito .91 4-stroke would also give you many years of strong, reliable flying service. They're all very good engines, and with the Magnum on sale right now for $149.99, it must be really tempting to try another brand. I certainly can't talk you out of it, I own 3 O.S. engines and they've all been fantastic, but so have the Magnum engines I've flown.

Whether you want to stick with O.S. Max for quality, reliability, and best-in-class warranty service or try the Magnum and know you're getting a really good engine while saving $100.00 is a personal choice that only you can make.