Take measurements of your alloys and then trawl e bay photos, local breakers etc and find wheels from another bike that look like they will just about fit and work from there .
Rear wheel spacers are simple , my ccm404 is running a dr350 rear wheel as it has a cush drive and is easier on the chain and transmission, took a couple of spacers and an hour to get it to fit .
Similarly my 1982 gsx1100 katana is running much later 90's era gsxr 1100 wheels .
Use info from the from online sources such as jt sprockets to give you an idea of rear sprockets, for example numerous CCM, KTM, Husqvarna bikes use the same rear sprockets so the hubs are all similar.
If you find a bike with a spoked front wheel and a similar sized front disc as your 310 then calipers/discs can easily be spaced out to fit and if axle sizes differ then front wheel bearings can be changed .
Look at bmw's own f650 funduro it has the same size front disc as the 310 gs and its rear sprocket is a JT sprockets JTR5 same as the funduro and numerous Aprilias etc
Talon will make hubs for a price , but is you can find a spoked hub that is near enough any machinist can modify it and you can have it rebuilt with a suitable sized rim. I modified a rear transalp hub to fit an old xl250r to get a rear disc to match the transalp front end I fitted to the same bike .
Buying from rally raid etc you are paying (and rightly so) for their development time and costs