If the tank has varnish and such in it, I find lacquer thinner or acetone will dissolve it, but it will take time.
I guess I missed the part where it was mentioned it had "varnish" type deposits.
Acid does better on metal not a "varnish" situation.
It will eventually work in the varnish, but prolly eat the tank also/first.
I wish you luck they can be difficult to clean but they are so small compared to a 20 or so gallon vehicle tank. :BlahBlah:

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In addition to varnish, there is rust (at least looks like it). Perhaps I should try lacquer thinner THEN Muriatic acid.

I've been using vinegar for rust removal, it works slow, but it does the job. For varnish and paint removal, lacquer thinner is my go to. I've soak carbs in it numerous times. ( tryclorethelene use to be the best, back in the day)

Perhaps you misunderstood the concept which is not only to seal leaks but also to coat the inside of the tank with a lining that seals the rust and crap away forever and is impervious to anything that you might put in there. Again, not promoting, just trying to help.

Other videos on that website about sealing "motorcycle" or sub-5 gallon tanks may be more informative. Granted, the kits they sell are expensive but so are old used K Series Kohler gas
tanks.

Thanks for the clarification. My goal at this time is to get it running.

Found out this morning why the hydro release lever does not function properly. The "extension" outlined is broken off (missing) from the drive line cover mechanism. :(

Photo courtesy of 'the auction site'

That is pretty common for them to break. You can just hold down the two release buttons by hand to push the tractor, providing they are not stuck from sitting.

Oh boy....weak link. I can't hold down the buttons and pull it out of the confined area by myself. That is why I am trying to get it running so I can drive it out. At least that is my plan.

What is a reasonable price to pay for a good used cover/lever assembly? How difficult to find? Doesn't look like an easily repaired item.

No need to buy the complete setup.
Charlie at CCS, up top, will have just the flapper.
Easy fix

Thanks! Called left message and also sent an email. How does it attach? It looks like it is spot welded to the lever shaft.