I had the Epsilon (7 1/2 LB) with a OS 91 and it flew great, I did build the kit. Most ARF's are to heavy for any real performance with a 90, a friend had the Excelleron with a Thunder Tiger 120 and done fine but weight was more like 8 1/2 lb which is about right for most ARF's.
The Epsilon is a 60/90 don't really like this lable as all are 90 size, and most need a 120 to really fly them with good power, go by weight and size.
60 engine = 6 1/2 LB max
90 engine = 7 1/2 LB max
120 engine 9 1/2 LB max
Steve Maxwell