Another benefit in my view of the earlier ETZ250s is the larger frame and 18″ wheels front & Back so you only need one spare inner tube. I don’t really like the ETZ250 power characteristics much either. I am told the porting on the 251 versions is milder but I cannot say I really noticed this when I have ridden one.
A few years ago I modified an ETZ250 by fitting a TS250 top end. This is a bit tricky as the stud spacing for the two is different and the ETZ barrel is longer needing a 15mm spacer to bridge the gap. But that does provide a means to sort out the stud spacing. It worked well enough for me to use the bike for a year or so including an E2E ride. Later the engine was fitted into a Trophy and sold with that bike.
However, by a curious quirk of fate it came back into my possession a couple of years ago and is currently fitted in an ETZ250 special I am working on.
It would in truth have been a lot easier to simply fit a TS250 motor into an ETZ250 chassis, they drop straight in, but at the time I wanted 12v electrics and the oil pump. On reflection I may very well fit a TS250 motor to my special with a Vape kit and accept the pain of premix.
Somewhere I have a drawing of the spacer, and its somewhere in the back numbers of MZ Rider – about 2009 I think.
I think the suggestion to try a Supa5 first is sensible, you may find you don’t really notice much difference enginewise. But you will probably notice the cramped riding position. My bike has a raised seat and dropped footrests to make it more comfortable.