Adler belt source?
 

There's an Adler near me, but it needs a new belt. Has anyone found a source for them?

Also, it needs a bobbin holder and I think it also should have a race plate or something like that, which is missing. Anyone have any ideas for this?

I think there is one dealer in France or Germany who has the original type cleated belt, but I can't make a search right now. They were a bit expensive, but within acceptable if you can spend a bit time and money on a machine. I have seen German Adlers run with new rubber timing belts, so I guess there are options if you take accurate measures and do a bit of searching.

What model of Adler is it? I'm sure there was a belt available on Sewing Parts Online. If it's a vintage machine, i'd probably stay away from it due to the problem of obtaining parts.