RE: YS 91 AC
I'd think any of the engines you mentioned should pull a 60 sized plane vertical. How much does the plane weight and what prop are you using?
Both the Saito and YS are very nice engines and I really have nothing bad to say about either, but there are some diffs between the two. The YS91AC is still a nice engine, but it is an older version and has had two replacements (91FZ and 110FZ.) YS engines run better than any other glow engine I've tried, but they can be fickle to setup and they really run best on high nitro fuels. If you are not willing to put in at least 15% nitro and preferably 20% nitro, don't bother with the YS. I'd put a YS100FZ up against a Saito 120 any day, but I'm not so sure on the 91AC. My guess is the Saito 120 will out pull the 91AC by a few hundred RPM, and the Saito will be easier to setup and maintain, but the YS will have better throttle response and is less prone to fuel delivery issues. The decision to sell or keep is obviously yours. I hope my additional info helps you in your decision.