IO-540 you are certainly right about the Spit. The number of built in rust traps is unbelievable as I have discovered to my cost. Got a fair bit of practice in with the MIG welder though! It is usually the ancilliaries that give the bother with the wiring being truly atrocious. Regarding the Vauxhall Viva analogy I was once advised to fit the voltage regulator off a Mini (BL not BMW) to my Aerobat as it was identical to the original.......
Fortunately my Cirrus was rebuilt by Norvic a few moons ago and I have recently invested one arm and most of a leg in a top end and magneto overhaul by Vintech so the old girl is getting the best of care. I'm still nervous about flying over anything bigger than Grafham Water though!