Honda-SOHC

I wonder if a giant ultrasonic cleaner would work?

That is if they even make one that a tank would fit in.

Be careful though, the bubbles produced are hydrogen.
I did mine outside over a couple of days.

I have cleaned the tank on my RD350 with amazing results.

Wash the tank out with water then get a small tub of Bicarbonate of soda from the supermarket. Mix it with water then fill the tank to the brim, I use half the tub with full tank of water.

Connect a DC power supply, battery charger or whatever. Negative to the tank, and the Positive to a steel bar (I used an old Allen key) and put it in the filler neck but don't let it touch the tank.

Turn power on and watch it fizz, keep topping the water up and every now and then clean the steel bar. It will take a good few hours. Also swirl the tank around to removed any trapped pockets of air.

Best to do it outside as the bubbles produced are Hydrogen 

I don't have a before pic but it was so bad I was on the lookout for a new tank.

This is a fairly slow process, it probably took 12 hours or so.

Did you check oil level with dipstick screwed in or resting on thread?

If you grab the wheel end and pull/push sideways can you feel any play at the bearing/frame junction.
If not pump it full of clean grease and see how it "falls" then

No play at all.

Thanks Julie,

I've just disconnected the shocks and the swing arm moves up and down freely, I cant feel it binding or anything.

It drops to the ground under its own weight.

On my vin plate it’s says built Nov 73