Charlie,

Go with the YS, its pumped and very powerful, fantasic liner throttle reponce and very reliable (I have 3 of these engines). There was a review of the Magnum 91 FS in the RCME over here in UK, dont rember the figures now but the rpm was down on the YS 63 (for the same prop) on average by 1500rpm. I get 11500 to 12000 rpm on a apc 13 x 6. Its weight a little over 1lb which by the time you add a scilencer to a 46 two stroke is similar in weight to the 2 stroke.

The only flame out I have had with the YS is when it is out of fuel

Bottom line worth every penny.

good luck

Kris