Mad question but ive sourced a cheap 750 street fighter with a running engine, was thinkin about making a street sleeper. Has anyone heard this done before.

hi iv put a thundercat motor into a 3tj, cbr 6 into a nc23 and a blade motor into that nc23 when the 6 motor let go .basically anything is possible if you really want to do it .from my experiences its easier to change the full loom ,cdi and clocks .if possible change the front end the 400 forks tend to be a bit light for the heavier motor also you can uprate the brakes when you change the front end.
the main problems are fitting the bigger motor between the fraim rails you will need a grinder .also i think you will have problems fitting the airbox under the 400 tank. mabey someone else will have zxr experiences and can add some info. personally i cant see it being that much harder with a zxr .

anything is possible, and it can be done, i've seen someone stick a CBR959 'Blade lump into the 400 frame - apparently no grinding of the frame needed to be done! however, if i were to do a heart transplant of a bike and the frame needed to be grinded then i wouldn't bother, you'll only weaken the frame.

just to clear up the cat into the 3tj was a straight fit as was the 600 motor into nc23 ,the blade motor i used was a early one as it was a few years ago i did this and the frame didnt need to be moddified too much just the arms that the head fits up between is too narrow so i guess a later model motor would be smaller and easier .but as the previous poster said anything is possible but modern 600 and 1000 handle so well it is hardly worth the bother unless you really want something different. just to add all the motors i changed were carbs .