If new tanks are available you can certainly go that route. If the tank is only rusty inside & not rusted through to the point of being structurally unsound it can be cleaned & coated as I noted. About all of the parts vendors offer clean & coat kits that use phosphoric acid as the cleaner and their choice of coating as the liner. I personally prefer using Muratic acid (the same stuff they put in swimming pools) as the cleaner and RedKote as the liner. The pros & cons and how to do this process have been covered here a lot. If you use the search function at the top of the page you'll get a ton of info you can use.
As for your tune-up questions. It sounds like you need to kit the carb. You can probably find the detailed tune-up info
here but I'd strongly suggest you invest in a shop manual. You can get them through most of the parts vendors and on eBay. Good luck.
