From memory, the belt is a spring steel one. Like a long small diameter spring with the ends joined together.
A possible source of supply would the garter spring from a lip seal, as used on cars and engines. Possibly a local garage could find one for you.
Interesting maths exercise for you. Measure the length of the belt required. Then work out the diameter of the seal that would provide a replacement spring.
When in place, it should not be so tightly stretched that the coils start to open up appreciably, but just tight enough to transfer power from the Flywheel to the road wheel.
If the spring that you get is too long, it can be shortened. You need to find the joint and cut the spring just away from the the end that tapers, on the side that does not taper to a point. The bit of spring left over can then be unwound frond from the tapered end.
The other end can now be cut to right length. The join can be remade by twisting the cut end, so that it "unwinds" a little (But not so much that it stays unwound ) before putting it over the tapered end and letting it "screw itself onto the tapered end. Hopefully,, by now, you will have a suitable belt.
My memory of Mamod engines is that they tend to dribble water. As a boy, many years ago, I could never cure mine!
Howard