You can get the base gasket from honda, but it would be spendy. I would replace all the top end gaskets, as you arent supposed to re-use the head gasket anyways, especially if its who knows how old. The head and jug can be removed in the frame, but it is easier to work on the motor on a bench if you ask me, but its your call. One place you can get a top end gasket kit would be at denniskirk.com ,, I usually have good luck with them. Otherwise, you can check ebay, or a local cycle shop in your area should be able to order it for you. Good Luck!
THE 80's RAGE! Currently owns:
81, 85 Yamaha Tri Moto 125
83 Yamaha Tri Moto 200 x2
82,83 Yamaha Tri Moto 175 project
85 Yamaha Tri Z 250
84 Kawasaki Tecate 250
84 Honda ATC 70
85 Honda ATC 350X
84 Honda ATC 250R
(2) 84 Honda ATC 200x
82, 83 Honda ATC 185s
81 ATC 200
Lots of other parts and bikes