Trust me guy's on this, put a OS 91 fx and you want go wrong. The OS 61 fx and 91 fx is just about the same size and weight. I think the 61 is 19.42 and the 91 19.44. Both engines are build on the same frame but the 91 is a little taller and has more power for the money. Now i have seen two of them fly, one with a 61fx and the other one a wren 44 turbine but the Little BobCat can take a some more power. I have one that we are going to put a PST 600R (turning it down) and we also have OS 91 fx sitting around that we are thinking about tring first.