At the very least, split the cases and clean it all out and fit all new seals. Fortunately RD stuff is everywhere. You can get a seal kit from ebay but a great source is Economy cycles in Illinois https://www.economycycle.com/ John the owner is a great guy and all he sells basically is Yamaha twin stuff.
You can also buy seals from your Yamaha dealer and they can probably supply gaskets as well.
My fear with that crank would be that it could be rust pitted inside the big ends. The "right" thing to do is to fit a new crank or send it to a crank guy and get it rebuilt. Chuck Supertune Quenzler or Lyn Garland are good. So is Bill Bune in MN.
One possible approach would be to wire wheel the crank wheels and flush the big end and bearings out with copious amounts of kerosene or WD40 and hope that it lasts you a season or two. That would be a gamble, but if budget is really tight, that may be one way to go. Either way, fit all new oil seals and make sure it's all clean before you seal the two case halves together.