Using a spray of water when the engine is hot will help a lot. As for exhaust pipes and mufflers remove them and toss in a big fire pitt for a few hours, then rake them out and allow to cool. Most of the carbon would have burnt off and any remaining carbon you will be able to shake out.

G'Day Orion, you have done very well with the rebuild. I think it sounds just right so time to refit it back into the boat

Very nice VP

I did my apprenticeship at removing broken Gibb Head keys from the McDonald Super Imperial Crude Oil engine I did up not too long ago. Both sides had broken heads and were also broken off flush to the flywheel. So spent the next 2 days drilling out the metal using 1/8" diameter drill bits. Had a stuff up when one bit bottomed out on a taper on the crankshaft I was unaware of so that snapped off inside too. It was a true challenge to get these buggas out but I did win eventually. Then spent a bit of time using wet n dry paper polishing the crankshaft so when or iff the flywheel was going to move and come off there was no rust to hold it back. So with a hydraulic puller, a lot of tension and then heat they finally came off. Total time spent removing 2 flywheels was close to 2 weeks, a lot of skin off the knuckles, a hell of a lot of swearing and an ongoing apology to neighbours for my language. So I do feel your pain on this topic.
As for Gibb Head keys, yes there are many different sizes but now not all are made. Engine manufacturers used these keys because they did the job and were cheap to buy in at the time. My McDonald has odd size keys so had to mill them down in height. I am not a machinist but a mate offered me the use of his mill so no time like now to learn. I lost hours of sleep working out how to machine these down and keep the taper. The solution when it finally came to me in the middle of the night was to place both tapers together and this left the flat sides that are now parallel so was an easy milling job. In order to fit Gibb Head keys they nearly always need some filing using bearing blue to ensure the correct fit. Fortunately one needed a little drawing filing and the other was a bit more but not hard to do just messy.
38ac, where your pics of the keys 'naked'?

Ok, Some Lanz Bulldog and Field Marshall tractors use a similar ideaa for a cartridge start. There is a clip around the place on how to make your own using blotting paper and a solution with I think salt petter in it. They roll them up like a cigarette and put them in a combustion chamber then hit a shot gun type of cartgridge with gunpowder only and that kicks them off.

As much as India and the manufacturing companies of many things gets a ribbing from most people  I have over the years purchased some BSW combination spanners, ring spanners and a good selection of sockets. All of these have been given a good workout on many of our toys as many are British. I have no issues with the quality. A friend visits India every 2nd year and does a tour of many large Companies doing talks to the Staff about many things they do and how to improve. He claims the factories he visits are all spotless and very modern and equipment is the latest on the market.

Lovely job there VP and neat too. I was going to say that it is not hard to fit some copper tubes into the tank as a return line inlet but your idea works just as well.