Welcome! I think you will find that the 250 frame is longer than the 125 so the tanks aren't interchangeable.
Now see someone prove me wrong!
ES250 Doppelport, ES250, ES250/1, ES250/2,ETS 250, ES150, ETS150, BK350, IWL Pitty, SR56 Wiesel, SR59 Berlin, Troll............ and thats just the German two strokes! http://thecomeconcollection.blogspot.com/
Hmmm, that's a shame. Are there any different tanks that fit the 125 that leaves a smaller gap between the engine and tank? I love the bike, just the style I'm trying to do would not suite the tank.
Also, the seat I'm looking to put on is a cafe racer style seat, how would I go about finding one that fits?