Well, just doing the math, the 36 T on a 20" is the same as a 47 T on a 26" wheel
(of course rounded to the nearest tooth)....since I have not tried a 47 T I can only approximate by using my speed using a 44T and scaling back.....With the 44t I have
gone about 28 MPH (might be able to go a little faster but the engine winds out pretty good)....since 47 T yields about 6.4% lower gear ratio, I will scale back the speed by the same percentage and arrive at approximately 26.2 MPH....so with your gearing 36T and
20" wheel revving the engine fairly high (my bike with the 44 T has a digital tach in it and I thing I was pushing 5700 RPM....I'll have to do a speed / tach test again to see the actual engine speed but I am going from memory now).....26.2 MPH is far from the 40 I think you were hoping for....With the smaller wheel you will definately need a smaller gear if you are hoping to break 30 MPH without making the engine scream....or worse blow?
I don't think anybody here has had an engie give out due to high RPM's but I could be wrong.
Andrew