1. Check eBay, over and over, if your really lucky they might turn up, or maybe someone here is hoarding some, could post in the classified section. Also check out the Honda Hoarders group on Facebook, lately i have connected with a lot of guys on there and there is some serious parts hoarding in the dark corners on that group.
2. I am fairly certain that the earlier 125 drives fit, but the main thing is you need to check your front wheel, see if the wheel has the little tab on it that drives the speed drive gear, if thats not on your front wheel then you don't have what you need even if you do manage to find the other parts, and they aint easy!! The last Hondaline ATC speedo i saw complete with all the parts was $700 on eBay... And that was the cheapest complete unit. That's why i am building a GPS speedo to go on my ATC200 now.
3. Post in the classifieds section, check eBay, they are out there. You will probably end up with a rack that needs a little refurb, a good media blasting and powder coating or repaint. I have seen a few of the racks on eBay, but i don't know if the 86 125M is machine specific or if it shares with other models, that could make it easy or hard to find it.
Wish i could give you more info, but an 86 125M is like the holy grail for me, i have only seen one in person, been denied for years to buy it, and i have never had the chance to work on one. They are just not a very common machine. Great score though, post up some pictures of it and welcome!
---------------------------------------------------------
1984 Honda ATC200ES "Big Red"
1982 ATC200E "Hondie"
1988 TRX300FW "Project Quad" Still in progress....