From my experience, in particular:
1. Spark plug bodies are notorious for corroding due to moisture getting into the plug wells, so unscrew, clean and re-insert every year. A stuck plug in an alloy head is no fun.
2. Remove rear brake drums annually and check brake cylinders have not seized;, apply ceretec grease or similar under dust boots.
3. Lubricate rear spring leaves with motor cycle chain oil (one that is suitable for o-ring chains)
4. Spanner check every nut and screw, as screws in particular come loose
5. Check diagonal tie-rods from top of cross frame to chassis rail; I have had two break over the years and it doesn’t look obvious unless you give them a shake. (Do this every time you grease the king pins.)
6. Lubricate the pedal box and clevis pins annually