HI All

I am looking at moddifying the engine but not sure what to do, what mods have been done and are possible ie can a newer cbr600 head fit a FJ to increase the cc or cam gears any thing like that to help increase power / performance will help?

 

Not to be an idiot or anything, but I would not waste your time on any extreme mods for the F. For the cost of them, youd be better off buying a more modern machine. However, some simple mods you can do are:
Carb Jets
Exhaust
K&N Filter
Ignition Advancer

Also you could Seafoam the engine to clear out the carbon deposits and clear up Lost power.

Now, i would seriously not recommend it, but you could have parts custom made, but they may not work at all.

Ie, custom ground cams, as well, I believe you can order Pistons from JE Pistons, but you need about 10 different measurements to do so. (Wiesco also does custom orders, but requires a minimum of 20 pistons i believe)

 

I agree with tds. Big time engine mods on an old 600 aren't really worth it, imo, and custom parts are expensive.
It's certainly not an insult to you or your bike, but you'd be spending big money and time for minimal gains and possible reliability issues vs. just getting something newer.