You need to strip the bottom bracket out to find out whether the innards are in a better state than that ring. If it's just the ring, I for one have some lying around, and could let you have one. You can loosen and tighten lockrings with a hammer and punch or similar, but a lockring spanner is better, especially if you want the replacement ring to look good. There are lots around;
Park Tools and
Hozan are good, but a cheaper one may do you.
If you need a full BB, decent cup and cone ones are hard to find now. I'd probably try
SJS Cycles, but even theirs are nothing special. You need an exact match for the BB spindle - measurements vary. Although, if it were me, I'd be putting a sealed unit in instead.
I've mended loads of mudguards with an ordinary hand-held pop rivet gun. The rivets tend to sit slightly more proud of the guard than the originals, but unless clearances are tiny it should be fine.