You can probably get away with the fractional dimension, but you will have to get new rubber for the wheel set. But a single Speed Bmx Wheel is not as wide as a Multispeed 20" Wheel. You may need to change out the rear axle and add in some axle spacers to accomodate the rear Free wheel.

Another consideration is if the Spinergy rear wheel is one of the newer cassette sytems made for BMX, the swap may be impossible. I haven't dealt with Spinergy for over 10 years. I don't know their product that well anymore. Check with Spinergy to find out.