Hi Stuart,
I have just had the same problem with a 7″ Webb front wheel as fitted from 1927 thru to about mid 1930s. After a lot of measuring I have designed a conversion to ball bearings. This is only suitable for the hollow axle with knockout inner spindle used on Scott forks on Flying Squirrel Deluxe, TT replica, etc.. There is already a conversion for the solid spindle type used on Webb forks and I have a drawing for that if you send me a stamped addressed A4 envelope. (My address is in Yowl).Back to the hollow axle type……On measuring the cups I was surprised to find that they are apparently Metric on the outside diameter at 44mm so I thought that finding a suitable taper roller bearing or ball bearing would be easy. It wasn’t!!! All I could source is 44mmx17mm magneto bearings, and they were cheap at only £7-50 each at Rollerball Ltd. I haven’t tested the conversion yet so no guarantees, but again I can send you a drawing via stamped addressed envelope. Hope this helps.
Regards, Brian.