If thats the back sprocket you are after, there are a range on offer on ebay
https://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=p2334524.m570.l1313&_nkw=Yamaha+rs100+rear+sprocketSome of them are so inexpensive, you'd wonder about the quality. ?
Note that the number of teeth is important, best check that what they offer is the same as you have.
And if your present sprocket teeth are really worn, its best to also replace the front sprocket and the chain as well.
No chainguards on offer there, but if you check every week one is sure to turn up.
Enquired of your local Yamaha Dealer ?
There are also plastic universal ones, how well they fit, or could be adapted you wouldn't know until you tried.
Any motorcycle breakers/wreckers nearby ?
Or even mail order ?
Hopethishelps.