Mk1 Conversion to Electric Starter with Norton MK3 OEM parts.

... keep the gear lever and back brake in the original (proper) positions? When I had a Commando this was something I pondered but moved the bike on before doing anything about it.

There is no need to convert to left-hand shift. That was a U.S. market requirement unrelated to the starter.

The Mk3 crankshaft has wider-set main bearings in addition to the longer mainshaft for the starter gears so will not fit directly into earlier cases. In addition, the mounting flange and stud positioning differs. This is really only a viable conversion if Mk3 850 crank and cases are used too.

There is someone in UK who makes add on starter conversions for BSAs, which you can find on the net. This maybe a possible route to go for?

 Have a scheme worked out,  parts made on my lathe and mill and fitted, send me your email address and I can send you pics. Uses a starter with internal planetary gears and 2 sets of sprockets to get the right torque to spin the engine. Starter is from a KLF300, sprag and one set of sprockets from GL1000.

I agree with Richard, only doable with mk3 cases and crank

You could do to study how the CNW conversion does this. The earlier cranks aren't long enough  to use MK3 bits. You need the mk3 crankshaft as it has the extension for the sprag drive gear. This is why CNW use a 20mm wide belt instead of the triplex chain, to make enough space to cram the starter components in. If wanting to stick with a chain you will have to use a duplex chain. You are still making a lot of custom components which is getting away from using 'genuine norton parts' Have you a mk3 that you can examine and measure? From what you say it sounds like you have yet to see the insides of the mk3 primary case.

Also the mk3 primary cases have the gearbox fixed in place as the gearchange required it to be fixed for the crossover shaft. 

Are you wanting a lefthand gear change or a righthand one?  The right hand one would be easier in this instance. Would still mean modifying the rear of the mk3 primary case to the earlier moving gearbox arrangement.

Let me know if you want to sell the mk3 primary cases.....