In theory this should be easy as the ETZ251 used basically the ETZ125/150 frame. However it was not quite straightforward. I once fitted a TS250 engine into an ETZ125 frame (not sure why now – just to see if it could be done I suppose). Anyway, from memory the tricky part was modifying the offside engine plate (you must use the TS/ETZ250 type) to give the correct width and offset. I think I welded an inner track from a 6306 bearing to the inside of the engine plate. You will also need an ETZ251/301 exhaust pipe else it will hit the mudguard on full fork depression.
I cannot remember the detail now but I have a vague recollection about swapping the s/a as well.
Of course there is also the little matter of telling the DVLA and your Ins company what you have done!
HI I have not done it but I have a least 5 etzs125 and as many etzs 250/251. The 250 is a larger moteur and the rear wheel and swing arms are different. The etz 250 wheel is also wider. The difference is about 2cm.It is a lot of work for nothing. Both the 125 and 250 are nice bikes.Sell ,the 250 moteur and buy a 125/150 moteur. Many of the other etz parts are common but not the moteur and the rear end in general. Joseph Schock in Paris