you were those things way before I ever came along...
The thing about old 1970's-80's Japanese bikes is that many were testbeds for really wild technology long before they made it to their cars and some can be fun and quirky and cool and some can be unmitigated disasters. But they all look the same and people generally regard Japanese machinery as being indestructible, which is not true.
The one thing that I find that frustrates car people who are looking to get into bikes is that during that era there are very few shared mfg standards. A good examples of this are wheels: with cars it's basically a lug pattern and offset and while there are a few to choose from they are easily identified - and that is how you can mount jeep wheels on a ford, bmw wheels on a honda, or buick wheels on a mopar fairly easily. For bikes, every wheel is proprietary and while you can sometimes get bearings to make it fit an axle everybody used different rotor offsets and thicknesses, different spacers, etc....so a wheel swap usually involves somebody machining something. Internally there are some standards, like honda using the same rear axle diameter on everything from a 1960 cb750 to a 1998 cbr600f3 but it wouldn't be the same for suzuki, kawasaki, or yamaha.
Even among same models you run into significant interchange problems - for example a 1976 cb750F and 1977 cb750F use the same engine and chassis, however they use different wheels, brakes, and forks that are not individually interchangeable. The 76 uses a 6 lug stainless rotor that is really thick, the 77 uses a pair of 5 lug thinner steel rotors. The only way to upgrade a 1976 with the better 1977 brakes is to swap the entire front end over from the top clamp to the tire, and even then you change the geometry slightly since the 77 uses a 60mm fork offset and the 76 has a 50mm fork offset.
Something like the RE5 has parts bin compatibility with the GT750 and GT550 suzuki two stroke triples, so getting parts like brake pads, fork seals, etc are easy (but may be getting more expensive since those bikes are getting collectible). But that rotary engine? yeah there are no parts for that engine. I suppose you could take the chance that corrosion has not ruined the apex seals, but I wouldn't gamble for $1500.
Want a cool bike that needs some TLC?:
http://dayton.craigslist.org/mcy/5794758015.html
Same mfg, 2 years newer. roller bearing crank, same money. Parts are widely available including racing parts. This was the bike that convinced pops yoshimura to stay in AMA superbike racing after the butler and smith bmw's stomped a mud hole in everyone's ass and walked it dry in 1976. By 1979 he had Wes Cooly on a gs1000 dominating AMA superbike. If you look hard enough there are turbo kits and you can run up to 12 psi on a stock engine, provided you can keep the turbo oiled (the old turbo suzuki drag racers used to use cessna fuel pumps to pump oil because the big zook has a HVLP oil system that makes 15psi at redline). That's a heck of a lot of value for $1500 compared to the RE5, and let's be honest, the only reason these are not $1000 more expensive is they don't say honda on the tank and nobody makes a bolt on cafe racer seat for them, even though they handle better, are faster than a SOHC honda, and are more comfy. I have owned three of them, and I was stupid for selling any of them.