If you are in the buisness of making money from those machines if your one multi purpose machine dies you immediately need to repair or buy a new one. If you are just tinkering and time is not money then a multi purpose machine is ok.
I am assuming you are not trying to build a show quality mower, I would say just paint it with some good paint. If you want it smooth looking you could as suggested try sanding. Another option is apply a skim coat of body filler or glazing putty prior to sanding.
To counteract the weight of the new compressor unit if you cannot move it inboard closer to the motor you could also move to motor out the opposite direction. of course you would need a longer drive belt.
I have an old Quincy 210 if i remember correctly that was originally siezed from rust in the cylinders took the head off soaked the valves in evaporust and poured a little on the piston then using a piece of wood and a hammer hit the tops of the piston got it broken free made a head gasket and...
A centrifugal clutch requires the ability to control the engine rpm how do you intend to control that accurately for demand when needed. I have a alternative a timer that shuts the engine off after say 10 minutes of continuous time at a specific pressure but restarts when the pressure drops...
I would second white lithium grease but would caution not to use too much as it could attract dirt causing rapid wear. That is probably why the gears were plastic. In my best guess.