Hi there, the pawl behind the cluch basket,. maybe need a spring, [ broken or fallen off] ,take off the side cover again, and get some one to select gears,[ engine off] see if its grabbing the selector drum, its a claw,
turn the back wheel by hand to change up the gears,
Your on the right track,, what you need is a parts manual, to see if the rod is cross reference with other models , ie-= DT125/175 etc..
CMSL, parts europe may have them, search the site,
https://www.cmsnl.com/yamaha-rs100-1976-usa_model8571/partslist/#.W-2_E-j7RqM
Yeh NSD, been the best summer since 1976, Did a few longish trips, ticked of the list, ,but our summer ended fist week in Aug,, i noticed you lot down south had high temps right into Oct,,,
You can fit a slightly wider tyre on than standard, it pulls the tyre into a slightly more peaked shape, they say it makes you tip in quicker on corners, well i fitted a 190- on instead of a 180, and noticed no difference ,