If you can do it without chopping the plugs off the end of the loom then you always have the option to replace them easily -- or at least, don't throw the plugs away if you chop them and leave some decent tails so it can be spliced. Be aware, some bicycle speedos don't read reliably over about 40mph - I had one on a motorbike a few years ago and it got all confused doing things like displaying 12mph when I was doing 70.... I would only see a 'need' to start changing spark plugs and stuff if you run a wireless rf speedo and it shows interference.
Having (or not) an odo isn't that much of an issue imo. I'm pretty sure there's not many people with a 20+ year old 90,000mile bike who can prove that mileage 'beyond doubt'. Clocks are easy to change, what's to say any possible replacements had a higher or lower mileage, or got reset to the same?
As rbm said, you'll need to run the magnet and reed switch that comes with the bike speedo, probably mounted on the front wheel/forks as well.
As for dummy lights, with the exception of the charge light (which if you really don't want on display you can stick a bulb anywhere and hide it) I would only class the ones legally required as 'critical'. Here, that would be main/high beam and indicator repeater (if indicators are fitted). Everything else is a 'nice to have', my old BSA had a main beam light, and that was it - I got on with it fine. Oh, and here as well the speedo should be backlit/illuminated, but I've never been questioned when I was running one.
