Try putting some washers on the shaft as well as the bearing,to make sure the clutch is fully home.It needs to be very tight .If it still comes off when running,then the crankshaft may have been damaged when it spun off.
A member of my section, [Yorkshire] had a keyway machined in his crankshaft,to prevent the clutch coming loose,but the cost was not far short of buying a new crank.Don’t know about the 250/1,but a new crank for my ETZ 300 was 90 euros from Ost2Rad,[currently,101 euros],it has now covered 12000 miles without any trouble.