The tires are still available for both the YG1 & YG1-K.
I once fitted a 300 x 17 on the rear of a YG1-K. It handled VERY well though the license plate bolts had to be replaced with button head screws to keep from rubbing the taller tire.
The YG1T & YG1-TK models use 250 X 16 rims. Cheng Shin also makes off-road tires in the C755 series in this size. The additional wheel clearance will easily allow for a 350 X 16 if you like climbing hills.
IRC Tires is another manufacturer who still makes vintage sizes.
A new Yamaha YG1 tank is probably worth it's weight in gold. They haven't been manufactured since the early 70's and it would be a very rare find. Your best bet would be to either Kreem your old one to plug up the holes, or find one at a junkyard in decent shape. The tank knee pads (I assume thats what you are referring to) are still out there if you call enough older Yamaha dealers. You may have some luck with Speed & Sport too.