Thanks Cynic,
You are right about the chain routing - I hadn't followed the path. If I can't find a decent blade for my grinder I will clip it with the bolt cutters.
I think I found the standard wemoto chain that you mentioned ( www.wemoto.com/bikes/Yamaha/ybr_125/10/chain_did_standard_grade )
Slightly concerned by a note in the Haynes manual that you should "never use a clip-type connecting link instead of a rivet-type link, except in an emergency" as the DID chain comes with a "spring link" ... are Haynes just being overly cautious?
Once I get a few bits I am going to dump the oil, swap the plug and swap out the old petrol and get it started. I expect it just needs its throat cleared and a bit of a use of the brake disc but we will see.