Rim sizes were based on the beaded edge tyre size back then. There were quite a lot more tyre/rim sizes available than there are today.
(I've got a new, old stock Dunlop Clipper 26 x 2 3/8 in the shed), god knows what size the rim should be!
Basically you have 2 choices, 26 x 2 or 26 x 2 1/2. I would suggest 26 x 2 1/2 is most appropriate - I have an early 20s 250cc Triple H that runs 26 x 2 1/2 tyres and rims.
The 26 x 2 size is more appropriate for veteran machinery, as tyre sizes started smaller (descended from push bike sizes) and grew as time progressed.
26 x 2 1/2 beaded edge tyres have had a supply problem recently (Ensign brand), but I believe it may be past or at least coming to an end.
The belt rim will be more of a problem. There is currently nobody in the UK making replacement belt rims. I've been investigating with a chap in West Yorks who used to be a supplier, but quit when the quality of deep drawing steel sheet dropped off, he claimed that 5 out of 6 rims ended up as scrap!
Still trying to coerce him into making an odd one for me, but as practically every belt rim is different, they are very labour intensive for creating the formers to help generate the shape.
Good luck with the rebuild, hope to see it at Banbury one day!