I happen to have a 2007 ZX6R front end here and was wondering if it would fit on an F4i without any significant fabrication. Never done anything like this before so I'm mostly just asking out of curiosity.

 

I'm fairly sure it would. Go to allballsracing.com and look for conversion bearings and it might go right on. You can enter your bike's info and it will tell you what bike's front ends fit and if you need different size bearings. Stem needs to be the same length or you need to be able to press the F4i stem into the lower zx6 clamp.

You can find tons of info on stunt forums - I don't think any of those bikes have the original front ends on them...

 

Thanks for the link- that's a pretty cool tool. Unfortunately it didn't list that year of ZX as compatible. Oh well. I don't really have any reason to swap it.

 

If you already have the fork you want to use you can measure the stem (I.D. of the bearing on the new fork) as well as the O.D. of your existing bearing and order what you need. All ***** has helped me with this and there's lots of industrial bearing suppliers who can, as well.
While this modifications is usually possible, things to look out for are length of the new stem vs. the old and location of steering stops. These can be issues that will require a machinist to resolve.