The MZ 250s have a M50 x 1.5 exhaust port thread.
I would probaly go bronze for a replacement thread insert rather than steel. I have a couple of barrels with duff or iffy threads & considered boring out the port & re-threading it M55 x 1.5 for an insert but, as my workshop capabilities are capacity limited, that would mean buying an M55 x 1.5 tap (can't fixture & swing a cylinder casting on a Myford to screw cut the thread !) - least expensive from China on Ebay is around £100, which is a bit steep & I don't fancy trying to make one.
Burwins in London have been MZ specialists for many years, so they may be able to offer the service - lousy website last time I looked though, seem to want you to ring them. Might be worth dropping Phil Speakman a line as well (MZ Shop), as he offers some engineering services. No personal experience of either company though & no idea of costs if they do offer this.