Hi Colin,
Posted this a while back and it may be of help - I use 'HPC Gears' at Chesterfield
for anything gears, pulleys and belts wise, parts are usually in stock and available from their massive warehouse site -
provided one understands the technicalities involved !
Recently replaced a slipping Meccano brass pulley and rubber band/belt two stage reduction drive on a 30" stern wheel steam launch,
with more substantial toothed belts and pulleys from HPC.
Lots of technical info on their website, most of it going straight over my head - I'm not an engineer,
therefore I took the model along and HPC's technical guy went into action with micrometers, vernier and calculator,
measuring shaft centres and pulley diameters etc.
He returned from the stores half an hour later with the exact components -
one pulley requiring minor drilling out to the correct shaft size, excellent service, but not cheap!