You don't mention which engine you put into that little electric Revolver. Just like real life, the weight of the new power plant should match the weight of the one being replaced.

RealFlight takes a bit of searching to get everything changed. I'm guessing you saw the power plant options and changed from electric to fuel.

I did that just now and the plane I was fiddling with went from 6.712 lbs to 8.163 lbs. It went from a Kontronik KBM 42-20 motor to a ST.90. They show the power plant weight going from 6.2oz to 25.9oz.

I think you'll have to consider everything to do the swap in RealFlight that you'd have to consider in real life.