Regarding the battery & i guess you're way past that [page behind 'me'] if you have a charger & the battery is dry but doesn't look sulfated, you 'could' just top it up with tap water to the full mark & charge it. Wouldn't worry about using distilled water considering it's likely past saving. Just what i'd do to try.

I wouldn’t use muriatic acid nasty stuff you could take it to a radiator shop and they can boil it out just make sure it won’t peel your paint unless you’re going to repaint it 

After the Evaporust I rinsed it out well, dried it quickly with a heat gun on low, then misted the interior with WD-40 through the fill hole. Many months later, there's still no rust inside, though I try to keep it close to full. Not saying this IS the method, but it worked for me.

Evaporust is by far one of the best methods.

I would NEVER line a tank unless it absolutely needs it... i.e. it has pinholes in it or something.

Adding splash of two stroke oil to the gas every once in a while helps keep rust away.

Metal etch is awesome too. That's what I used exclusively before Evaporust... and yes the phosphate coat it leaves is great.

That's an excellent idea Mark, a little splash of two-stroke oil in the tank. I'm going to do that.

It's them oil pumps ya know. Never had rusted tanks on premix [or did we].

Was able to have a new battery ordered by a local shop. Thanks @MARKT for the idea. They filled with acid and charged it, so should be good to go.  Question on the routing the battery vent hose. How do I ensure I do this properly? I reviewed the manual, and the cable routing diagram isn't very clear. Can anyone provide any photos as to where it should go?

Also purchased some Evaporust from Amazon. I'm hesitant to mess with sealing it, so I think I'll do what some of you suggested...oil, wd40, or something similar and then get gas in it.

Plan to grab some Yamalube 2S tomorrow locally, so should be good on the oil front.

Daughter and I are planning to tackle the carb tomorrow night. Looking forward to finally getting our hands dirty  .

Important [very] to be sure the breather tube isn't trapped or blocked. Clear tubes that i've ever seen that came with a new battery [thinking Uasa Battery] have a small split about an inch from the top maybe half an inch. Should have one top & bottom so you can't get it wrong whichever end you connect to the battery. Important because the battery could blow apart. I saw that happen at work [thankfully not me] where a new battery was put in a new triumph trident [50 years ago] & don't think they took the cap off the breather [let alone fit the breather tube] which must've been on tighter fit than i'd think. Battery had just been filled so i guess was fizzing pretty hard. Next thing it was all over the chrome muffler on the left--& everywhere else. Stuffed the chrome & likely more. Washed it down but too late, damage done. By memory they might've put it in then rode it around the block & had blown in the short ride. So be aware, that's all--big 12 volt battery=big mess.
Just to add, i'd think [hope] even if it comes with a black plastic tube it should still have a safety split.

I’ve only been able to find 4 stroke wet clutch rated oil. Is this still ok to run in the transmission?