I am a bit outside of chorus, my father is a mechanic so its a family madness, but with such a long trip in mind and with this high mileage, if you have time and money I'll go for a complete engine check:
Check and clean/rectify/sobstitute: valvels, head, cylinder, piston, piston rings, clutch, gears and gear selector.
Change all the fluids and check every pipes for cracks.
Check all electrical, sobstitute if necessary, spry all connectors/connections.