G15 CSR With Mk3 850 Engine

Looking for some expert help, tribal knowledge or helpful advice

I am rebuilding a '69 G15 CSR that at some point had a 1975 Mk3 850 cc engine (I think) put in it. On the original engine the inner primary case attached to the crankcase with 3 bolts. The later engine had 4 bolts. You can see from the picture how the PO made this work. I think I need to source another inner primary case, weld the 3 bolt pattern closed and drill a 4 bolt pattern?

Does anyone now if the inner primary cases on the G15CSR were common among other models such as the G80 etc? This might make the search for replacement easier.

Can anyone share any other obstacles I might come across in trying to repair/rebuild/improve this part of the bike?

Sorry if these seem really dumb questions, I am not super mechanically experienced and this is my first motor bike rebuild (first of any kind of rebuild project) and only my 2nd motorbike

If you contact https://www.accessnorton.com/members/ludwig.514/ he has done a superb job with the one he has.
It would be very similiar to what you want to do now.

S/N 328xxx is definately a pair of Mk3 cases. A Norton primary cover will not fit the G15 layout - covers are too long.
Primary cases for G15 are particular. It shared the outer cover with late model G12s.

You have some options:

1. Retain the simplex chain, the AMC clutch and (repaired) covers. Not advisable du to clutch slip and heavy chain wear.
2. Rebuild your tranny as per Mk3, fit a Laycock Norton clutch, run a triplex chain or a belt drive with the AMC outer cover spaced out.
If a slim belt drive is fitted (as CNW did), the spacer need not be very thich, maybe 5/8" only. Alternator stator will need to be relocated.
3. As (2) but you make custom primary covers. The easiest is probably to have billed covers made.

-Knut