Aero-kroil is like WD-40 on steroids.  I don't know how it works on castor, but on rust it's the bee's knees.  If you're down to penetrating oil and heat, the best thing to do is to be patient and keep trying.  You're hoping the oil will seep into some very narrow spaces, and that takes time.  Spray, let it sit, heat it up, tap it gently, then repeat once or twice a day until you're either successful or so fed up you're ready to damage things to get it apart, and buy new pieces.

(Or -- eww -- hand it off to an engine expert).

Looks like there's a bazzilion places that carry Kroil mail-order.  So if you don't mind doing that, you're all set.

Dennis is right on.
When flying RC, people will often put a plane into a fast vertical dive.
The prop can windmill causing the engine to over-rev, allowing the valves to float.
At that point, there will be interference and the piston can hit a valve, bending it.
A long time ago, I bought a used SAITO 40 with a stuck valve.
After dissassembling it, I found a small crescent-shaped mark on the piston crown and a bent valve.
Fortunately, the valve guide was not damaged and installing a new valve solved the problem.
The engine ran perfectly.
When you reassemble, you need a special tool to properly set the cam/crankshaft relationship.
The tool is very easy to make if you have a small lathe.
Let me know if I can help.

Bob Z.

Hi, Greg - good points.
The engine I was referring to was not the engine in the video but a very old SAITO 40.
I spent considerable time with one of the engine technicians at Horizon Hobby, the SAITO importers at the time, probably more than 20 years ago.
They also suggested that the issue might be due to over-revving, as they had a number of engines returned with similar issues.
Depending on the severity of the float, the momentum of the moving valve could carry it past the point of interference.
Regarding improper valve timing, that could well be the case. I just dissassembled the engine without checking.
In any case, with quite a bit of four cycle experience, I have never had a stuck valve due to varnish.
I have used straight castor, 50/50 castor-synthetic and straight synthetic.

Bob Z.

 Hi, Dennis.
You are absolutely right.
Straight castor and even 50% castor will cause serious carbon accumulations on the exhaust valve.

My recommendation is 20% total oil, and no more than 5% of that castor oil.
Many very successful 4-cycle flyers go with 20% full synthetic and 20% nitro.