Well, the answer will be yes AND no. Yes, to the compatibility - I mean, the seat unit (or the carrycot, for that matter, but that is not really of much importance for you at this point) is compatible with any other Teutonia brand frames. That means you could take the seat itself and click it on a different (older) Teutonia chassis (just not the newer Teutonia Trio since the new owners of that are actually the manufacturers of Emmaljunga.
On the other hand, the no will be for the wheels themselves - you cannot take wheels from a different - Teutonia or another brand - pushchair model and attach them onto the Fun frame. You would compromise the center of gravity, the sitting angle, and possibly the safety (in case the wheel(s) would randomly fall off, and the stroller would collapse).
As I see it, you can either:
- last with the worn-out wheels, if possible
- try to find any classifieds with the exact same pram and buy that just for the wheels/any spare parts just in case
try to find a cheap Teutonia pram and use its chassis with the Fun seat part (or just use the seat from that pram...)
- go for something new/or used, but different (I would suggest something even smaller with a 2yo child, to be honest, even if this is a quality buggy for sure - I mean, check something in the likes of a Nuna Pepp Next or a Quinny Zapp Xpress, the wider wheels will suit you, I think).
I wish I could help more, but yeah, it's an older system, as you know. And yes, it is almost impossible to find parts for such older prams overall, and in this case, even more difficult since the company changed owners and the whole portfolio.