You could try some domestic kettle cleaner BUT MAKE SURE it is suitable for aluminium kettles!
It sounds very much like restricterd passages though given the capacity of the engine, the ambient temperature meantioned and the size of the radiator the heat balance must be a bit marginal. For example; say 15BHP average output, that is 11KW and to get that you are burning something like 45KW of energy. About 50% goes straight down the exhaust pipe, about 20% is turned into useful power, the remaining 30% is probably split between general radiated losses and the radiator. The radiator has therefore to reject about 7KW to atmosphere and THAT is quite a lot of heat!
I’d not be too happy using silver steel for rivets if it is the same stuff that I know by that name. Silver steel is designed to be heat treated for hardened tools, punches and the like, in its annealed state its usefulness in this sort of application is to say the least dubious as the composition of the alloy is not designed for cold working. Do not however go into a panic, just keep an eye on things in case things start to go pear shaped!