Looks like a standard 385 - its high torque - slow motor with 5 poles - just what you need for the slow scale speed of a puffer. Low current draw too - gives nice long run time. Prop normally 3 or 4 blade . Try starting with 35mm Graupner plastic - check motor for heating after a few minutes - gives you a bit of a pointer - you will have to experiment as Glynn Guest suggests - take a handful of props with you to the pond. Anywhere from 30 to 45 with different pitches. Could use a test tank outside otherwise - even a slow running motor pushes a large volume of water - the indoor test tank would tend to empty quite quickly. Better halves seem to get a bit twitchy about that.
You can attach a fishing scale or similar and tether boat via it to any convenient tree etc. That will give a good indication of thrust produced - and you can quickly find out what the motor does heatwise.

Any ideas?

After doing a bit more searching found this. Would it work?
https://www.ebay.com.au/itm/1pc-R385-DC-6-12V-6500RPM-Long-Life-Carbon-Brush-High-Torque-Motor-for-DIY-Model/223269629521?hash=item33fbe82e51:g:GtsAAOSwsGpb91Nx