Welcome to MBc,
First it is important to know the speedometer is way off on the NE-r and the Ambassador. The tires have a 10% longer travel than the 26 X 2.125 tires the speedometer was made for. If you are going 37 MPH, then add 3.7 MPH, or approx 41 MPH. I just finished a partial upgrade on a NE-r and it hits 42 MPH on the speedometer [approx 46 MPH]. It went faster but owner wanted less noise and the modified WC-1 muffler insert slowed it down 2 MPH.
I can easily find more power [my Ambassador buries the speedometer or approx 55 MPH] if you need it.
Lighten the lifters, mill & re-work the head, re-work the cylinder, and use a .010" copper head gasket.
Have fun,