I don't believe the change to timing belts is necessary with modern polyurethane belts, which have plenty of grip. I'm pretty sure the Vee belts fitted to many SLs in the 1970s were fine too, but you can't get them any more. I still find, even with a 24 V motor and speed controller, it's most useful to change the belts to vary the speed. When I fit the DC motor to my U3 I will probably make a set of pulleys as well, to give maybe four basic speeds, but I have a scheme in mind to add a real back gear as well to extend the to lower speeds. I will probably use a 150 W motor. The usual type is a simple bolt on replacement.
For the drive belts I might try the small Vee belts fitted to many sewing machines, but that will mean having a tensioning lever in the drive, so it can be slackened off to move the belts around.
