Pop the engine cover off of on a humid night and see if there's light show of sparks going on when the engine is running. Cracked distributor cap would be my first guess. Next would be the usual suspects spark plug wires, rotor and coil.
We have had a number of customers with the issue of moisture in the distributor cap on the honda acty. You might try a piece of inner tube wrapped around the distributor cap to keep the water out. We have many new and used parts for the acty mini truck check our onlone store http://www.minitruck.ca/mini-truck-parts
