The gear in question is unfortunately a double spur gear.
I say unfortunately, as finding an “off the shelf” correct combination of gears in a double gear is very unlikely.
Although it is somewhat difficult to work out from the photos, I believe the two gears in question in the broken item may be as follows:
The larger gear - 25 tooth MOD 0.4, OD 10.8 mm, 3mm thick made from Tufnol
The smaller gear – 10 tooth MOD 0.4 splined shaft (OD 5.2 mm) – could be steel or brass (6mm long with 3mm protruding from the larger gear).
It appears that the smaller gear (splined shaft) has been pressed into the Tufnol and then deformed at the bottom end to fix it.
(If they are not metric gears then they might be 64 DP but my guess is MOD 0.4).
Tufnol could have been used for a number of reasons including cost, noise dampening, or as a weak link, to be the first point of failure in case of a fault.
I have checked the catalogues of 4 or 5 of the hobby companies and they do not stock this size.
Moving to the specialist gear companies again the double gear in this specific combination is not a standard part (similar sizes are £30 plus – so more expensive than a new motor).
You could consider joining two gears. I join my own brass gears.
You can get the larger 25 tooth gear in for example brass for about £10 or plastic for about £3. Tufnol is available but prices are not given in my catalogue. (- prices exclude p &p)
(I can’t find availability/prices for double gears of this particular size but).
The problem though is how to fix the 2 parts of different materials. One possibility might be to use a brass 25 tooth spur gear and drill out the centre and insert the spline shaft section from your existing gear (having cut away the Tufnol). You might then be able to glue or braze the parts but you would have to be very accurate with the assembly. The problem with this is that you don’t know why Tufnol was chosen (over brass or steel) so introducing a brass gear may not be a good idea anyway.
You could try the cheaper Delrin 25 tooth gear using the same principle but it might be difficult to glue and Delrin is not as strong as Tufnol.
A third possibility is to fix together 2 new brass gears (one 10 teeth with a boss, and one 25 tooth centre drilled to take the boss of the smaller gear). However as each part costs around £10 it’s probably more expensive than a new motor.
At least you know the specification and might get lucky, but sorry I couldn’t find you a catalogue part. I will keep a look out at the exhibitions – it’s just the sort of thing that you find in the oddment bins !!