Some stats from the internet http://www.bikez.com/motorcycles/yamaha_mr50_1980.php
Pretty much, the twinshock is the spit of the DT50m. The Later monoshock is, apart from differences in plastics, and oddly no nosecone on the MR the same as the DT50 MX. The engines are broadly the same except for one VERY big difference......
A fit and healthy twinshock with the 16mm carb will crack 60mph, i can vouch for this having been clocked on my old 50 in the stone age at 65, (at 60cc with a lot of teenage fiddling).
The MX, at least in the uk..... well...... wont, 30mph in the uk. Boooooooooooooooooo. Fit the m engine parts parts, (carb/reeds/inlet/ exhaust) and yahay, so guess where all the m's went.