Finding new bearings for the M18 did turn out to be a hassle; it's an obsolete size at 22x50x17 mm. It took begging and substantial sums of money to get the sets I now have; an overview!
On both the side valve (SV) and the overhead valve (OHV) models, a single row roller bearing was used at the drive side; similar to the one in the top picture. SKF order numbers were W6000, nowadays the bearing is known as RMC22L.
The SV used a single row ball race bat the timing side; identified by the SKF number W5162 but better known as MC22; the bearing in the middle pic. The OHV often used a double row self-aligning ball race at the timing side, identified by the number 11494a; in the bottom pic.
In the past people may have used whatever they could lay their hands on; my M18 had a single row ball race MC22 fitted on both sides which worked perfectly fine for a very long time.