Is there any reason why mechanics don't charge by the service, rather than by the hour?
For example, you could charge $100 for analysis. If you can't figure out the problem, the owner hasn't spent too much. If you find a different cause for the problem later, the owner already paid your for diagnostics.
You could then charge, say, $200 for lower unit cooling system overhaul, plus parts. $200 for cleaning the powerhead and other upper-unit cooling system passages, plus parts. $500 to overhaul the powerhead. $10 for an oil change. Whatever. Deciding what is needed is covered by the customer's analysis fee, so along with a diagnosis you propose a set of repairs. If the customer doesn't agree or can't afford you, they can walk away with an analysis and see if another mechanic can do the same repair for less (other mechanics beware, how much do you trust your competitors' analysis?).
I'm just making up numbers on the spot. To me the principle seems workable. You could charge seemingly outrageous fees for services and still undercut mechanics using the traditional hourly system. So why not do it that way?
s/v Laelia - 1978 Pearson 365 ketch
s/v Essorant - 1972 Catalina 27