i own a zen 90 and a 120 both of then are excellent planes. the 90 has a YS110 fzs with a pipe and the 120 has an OS120 surpass with pump. The 61 FX on the 50 will give you unlimited power. Those 50 size planes are designed around a 50 four stroke engine or a 40 two stroke. The ys 60 fz fits like a dream in it with a pipe.

Here are a few things you can do to solve the tiping over problems.
I had the same tipping over problems until i did this.
Remove the retracts and put a 1/16 to a 1/8 piece of metal strip or plywood strip on the rear of the retract,shim it up at the aft. then screw it in place. this will raise the back of the gear and will automatically tilt the wheels forward. Works great. solved most of my tip over problems

Also you are probably too nose heavy so move the battery back and if that doesnot work then add about two to three ounces of weight at the tail end of it.
This will solve most of your problems and you will have lots of fun flying this plane rather then fighting it from nose overs.
There is a review on the zen 50 on rcunivers review section check it out i am sure it will help.

Let me know how you did.