You actually have a couple of options:
Have a trumpet mouthpiece cut down to fit
Change the receiver (would be easy enough to cut down the mouthpipe at the same time)
Try flugelhorn mouthpieces (I am not sure that you will find a modern flugel mouthpiece with the correct shank taper)
In any case, the response of the horn would be best with a trumpet mouthpiece.
Although I really like your idea of transposing up, if it were mine, I would get it cut down to modern Bb. The first step would be to measure the length of the instrument and compare it to one in modern, western pitch. I can measure my Heckel and let you know. It has the same geometry and the bore size appears to be close.