As per Curly, BEWARE they breed at a rapid rate (I now have 13 assorted 500 in the shed)and ownership is a well known affliction which can be managed with a 12 Step Program.
The '77 TT sits a close second to the 1976 XT / TT as far as icon status goes. Most parts for the engine are still readily available through Yamaha has a lot are shared with the SR500/400 which has basically remained in production for the last 30 odd years. Having your Yamaha dealer check the "dead stock list"will often pay dividend with parts that Yamaha list as NLA, I have gotten a lot of part through Motorcycle Land in Ballarat that I found this way.
Les at Kedo Australia (VMX Unlimited) is a great source of quality aftermarket bit and have got a lot of my money in resent times. Speed and Sport in the US have also provided many model specific NOS bits that are now obsolete.
Enjoy the ride and keep us updated on progress
CJ