I've had a couple of Maxims. Your biggest problem with hardtailing them is probably the shaft drive on the 650s and 750s. The 550s are chain. It can still be done with the shaft, but you'll more than likely have to keep the stock stretch and use the swingarm as part of your design.
The 550s are 6 speeds and the 650s and 750s are 5 speeds. They are inline fours so they need to rev up a bit to make power. They actually are pretty powerful though and you can do about 120 MPH on them. They sound nothing like an XS650 though, so if you like the sound of an XS with drag pipes, you'll never get that from an XJ (Maxim).
http://www.xjbikes.com/ is a good forum if you are looking for more info.
Another problem is finding parts for the motor. You can get pretty much any part you can imagine for the XS, but finding certain parts specific to the XJ might be a lot harder. Engine and exhaust parts would probably be the hardest. A lot of other parts from the XS will work on the XJ though.
Keep looking for an XS if that is what you want. As winter gets closer, you should be able to find something. Check Craigslist every few hours for several days and I bet you’ll find something. If you are planning to hardtail it, you probably only really need a titled frame, engine, wheels and front end if you’re keeping that stock. So I wouldn’t worry about getting something that looks a little beat up as long as the motor is good. You will probably be able to sell a lot of parts off whatever you buy to help recover some of your costs. I’d look for a newer XS (1977-1983) so you get the better forks, clutch, starter, electrics, etc. Good luck! Keeps us posted if you find anything!
