I have had speeds between 45-50km/h with those russian engines made during fifties and sixties and their desing hasn't changed much since then. It has to be some adjustment problem. I can point out two possible problems - fuel mixture is wrong, probably too rich and ignition is too late or early (more like late).
You can adjust ignition with two screwdrivers - with one, turn cranckshaft and put other into cylinder to measure pistons position. Ignition moment is when contacts are disconnected. It is trial and error job.
For fuel mixture I had my own method - i turned fuel mixture screw out several turnes (don't remember how many, but to be sure that mixture is too rich. engine has to be warm), then starting riding wot and at the same time slowly turning screw clockwise (screw had bigger head and spirng for that purpose). When the micture is right, you'll hear and feel the difference.
And overheating doesn't break those engines, just let it cool al little after it's stopped, then start again and go.
I have them stop several times because of overheating during warm summer times riding wot.
