The following are useful resources for finding Japanese and BMW part numbers, plus some specification, sizing, and technical references. 

Japanese motorcycle part numbers and OEM replacement parts:

• https://www.cmsnl.com/
• https://www.partzilla.com/

BMW motorcycle part numbers and OEM replacement parts:

• https://shop.maxbmw.com/fiche/PartsFiche.aspx
• https://www.bobsbmw.com/store/microfiche/BrowseSeries.aspx
• https://www.ascycles.com/BMW-Motorcycle-Parts-Fiche

Massive database of motorcycle specifications:
https://www.motorcyclespecs.co.za/

Motorcycle tire speed ratings, load ratings, and sizes:
https://www.webbikeworld.com/motorcycle-tire-information/

Japanese fork tube diameters:
https://www.dotheton.com/index.php?threads/fork-tube-diameters-list.20950/

Japanese axle diameters:
https://www.dotheton.com/index.php?threads/axle-diameter-size-list.20952/

Free motorcycle service manuals:
https://www.carlsalter.com/motorcycle-manuals.asp

Wiring diagrams for select Japanese motorcycles:
http://www.oregonmotorcycleparts.com/diagrams.html