Ktrip,
I can't say as I don't own one, but there are reports of the small twins being restored in the UK, so I guess most wearing parts are available or similar parts can be made to fit.
The usual sources for engine parts in the UK are Norvil, RGM, Russell Motors, Alpha Bearings, Burlen Ltd., ....
Ball and roller bearing sizes will be metric I guess and should be easily obtainable.
Camshafts, cam followers and rocker spindles will likely have to be refurbished or re-made.
Some engine parts may be shared with the heavy twins range, and for these, Andover Norton is your premier source. You need to go by the 6-digit part number (use a conversion list where necessary).
Primary drive and transmission were shared with other models of the AMC group (i.e., Matchless G2CS, G5) and for these parts, the AJS/M Owners Group run a comprehensive spares operation. AMC Classic Spares may also have some parts.
Otherwise, you need to scout eBay for good s/h parts.
When digging deep enough, you will probably find a specialist in the UK specializing in these models. I suggest you contact the NOC (Norton Owners Club) for guidance. Have you searched this site? Member "Clinton4641" owns a Navigator and might provide some leads. You should also read the restoration report by german member "rivera".
Good luck with your project!
- Knut