I've had a few Systemdeks - IIX and IIX/900 - over the years, and that does sound like a belt that's just too small. If you're concerned about tension with the original belt, just turn over one end so that you have a twist in it, to add a little tension.
You only need enough bounce to cope with potential disturbance during play, i.e as many mm's as your floor is likely to generate as you step forward to move past that filler track that you really haven't been able to stand all of these years! The 'bounce test' should be a really gentle test...
If you've oiled the bearing, use a (tiny!) amount of the same oil around the spindle, smeared on with a fingertip, or wiggled into the hole in the platter with a cotton-bud. Next time you take the platter off to change speed, wipe up any excess. If you haven't oiled the bearing, do so, and go back to the beginning of this paragraph
Moth 1 is a Rega (RB250?) clone, I think, so have a look at the available Rega protractors here, or just do what lazy people like me do, and line up the cartridge square to the headshell and align the stylus with the end of the headshell.
Cheers - have fun, sounds like a really nice set-up