Will a new gen 2:1 exhaust fit my 88-07?
The short answer is NO.
The long answer is NO.
The stock 88-07 head pipe ID (inside diameter) is 1 1/8". The outlets are the same size.
Since most folks change a pipe for the "performance" boost, it doesn't make sense to replace the stock system for another stock system that is more restrictive and weighs nearly the same (the 250J pipe is HEAVY). Although you end up with a 2:1 system for cheap, you also end up with a bike that will not run as well or as strong as before.
As with most things, you could put a buttload of effort into it and make it work. At a minimum, you'd need to weld on the header pipe ends from the older F series to the new system to mount them to the head. Then you'd need to bend/rebend/adjust the routing of the newer pipe to conform to the F engine and plastics. Then of course you'd need to figure out how to mount the can.
You're free to try it if you'd like. Get a J pipe off eBay. But even if you have the equipment that would be needed to do the bending and welding, most people would find this to be one of those "not worth the effort" projects.
Oh, and have fun rejetting.
Here are photos of the 3 pipes together:
The one with the terrible DIY black paint job is the stock 08. The Muzzy has a VERY EXTREMELY LOUD unpacked, chopped racing can.