Forget it's cafeability potential or whatever...let's start off with the stock bike and it's many flaws. For starters (and while this probably doesn't matter if you cafe it) but the stock bike is uncomfortable. Kawasaki couldn't decide whether these bikes were standards or cruisers or super bikes so it has a really odd riding position which is unusually uncomfortable for just about anybody who rides them.
The vibrations that these bikes have are usually internal vibrations that cannot be smoothed out through tuning. This isn't the good vibration of a brit or honda twin, but the vibration of the balancer chain and balancer shafts loosing the good fight against the normal operation of the engine, plus it makes it 3x as hard to fix anything when you are taking it apart. They have notorious starter clutch problems also. When building this engine, kawasaki looked at their kz400 and basically just made everything bigger.
weight is a factor too as been mentioned by everyone else, but let's put this in perspective: it weights slightly more than a GT750 water buffalo wet and it makes about the same power output as a cb550 (at the crank it is 55, which means at the rear wheel is somewhere in the 40's). the one thing it does have going for it is that it makes 44 ft-lbs of torque which is a lot for any twin, but still not eunogh to make this lump feel sporty.
The chassis has a piss poor layout too. It pitches up by the gas tank to raise the front end up so they could have a "sporting" 28 degrees of rake with a lot of trail. The front suspension these bikes has a lot of travel and is pretty mushy. The frame is exceedingly heavy for what it is - compare a stripped one next to any brit twin and you will think you are looking at an ATV chassis.
the surprising thing about this bike is that the electrics are appalling on it. Japanese electrics are supposed to be top notch in the motorcycle world but not these bikes, they had everything from stator problems to fuse holder problems. For some reason, the company famous for pioneering CDI on motorcycles decided to go with a breaker point ignition on the kz750 twin. Normally breaker point ignitions are fine ignitions, but the kz750s tend to wear through the points faster than a lot of other bikes (I suspect because of the vibration). These bikes also were known to burn through ignition switches fairly regularly.
When I lived in new orleans, a friend of mine let one of the 1977 models follow him home. We worked on that bike for 2 weeks straight and still couldn't get it to run right. It was just a nightmare. We rewired most of the bike, rebuilt the carbs a dozen times, and were constantly replacing things that vibrated loose on the bike. We fought with the thing on a daily basis and in the end the bike was so determined to go to the junkyard we eventually had to stop fighting to keep her roadworthy.
My advice to anyone considering buying one of these and can't be talked out of it....buy a runner. Seriously, no projects with this bike. If maintained properly the bike will go for a long while (there are a few guys who have 50K-60K miles on them), but the moment you start to slack on the maintenance the bike will beat itself to death and multiple things will go wrong all at once.
