Given the crank damage, maybe your son's bike ought to be the parts bike, and that 85 ought to be the runner. Take it from me, I restored a 95 with parts from ebay, most sourced from one guy, and I think at the end of the day most of my bike was whatever bike he was parting out... I could have saved us both a lot of grief if I just drove to Iowa and traded bikes and handed him $500. :)
For parts compatibility, you can go to the dealer web sites and "work it backwards" from the online parts catalogs. It's a headache, but it works. You need to find the part on the diagram, find the reference number on it, then find that on the price list (which will include the "real" part number, sometimes the same as the reference number, sometimes not). Do that for both years, and see if that final part number is the exact same one. Doing this saved me a bunch of money when I needed a new output shaft... 80% of the pre 95 KDX transmissions are the same as the post 95 transmissions... including the output shaft. And the pre 95 trannies are a lot cheaper.
Service Honda has a nice online parts manual, and does mail order, so you can use them and you are ready to just order if the other bike parts won't work.