Just on the offchance anyone is left here reading: my old boss's son acquired a bonser 3 wheel market garden truck from laying 10 years under a hedge.
All seems pretty clean, rings and bore good, spark is weak but low compression caused by rusting around the exhaust valve. He ( 12 years old) has started grinding the valve but the stem has now bottomed out on the tappet and cam. Does he have to renew the valve seat or can he shorten the valve stem?
AJH