Try replacing the spark plug wires and plug caps. Make sure to use marine solid core wire, should be copper core. I had the same issue with mine, now it has bright blue spark on both cylinders.
The spark plugs on a 1988 model, 70hp Johnson outboard, should be gapped to .030in.
The 1957 model, 18 hp Johnson outboard, requires an Champion J4C, or NGK B8S, gapped to .030 in.
The Yamaha Pro 50 outboard requires an NGK BR8HS-10 plug, gapped to .039in.
The 1986 model, DT75 Suzuki outboard requires an NGK B8HS plug, or equivalent, gapped to .034 in.
The 1998 model, Yamaha 250 hp outboard, requires an NGK BR9HS-10 spark plug, gapped to .039 in.
The 1977 model, 70 hp Evinrude outboard requires an NGK BZ7HS-10, or a Champion QL77JC4 Plug gapped to .030 in.
The maker points in a 1967 model, 110 Mercury outboard, are gapped to .020 in.
Usually the occasional spark plug that needs to be cleaned and gapped.
Brand: NGK Plug No. BP8H-N-10 gapped at 1.0mm (0.029")
The 1996 model, 175 hp Evinrude outboard spark plug recommendations...... Champion Ql78YC gapped at .030
The 2004 model, 60 hp Mercury outboard uses an NGK BPZ8H-N-10, gapped to .040 in.
Assuming they are only fouled, and not cracked ceramics, or center electrodes badly burned down, yes - they can be cleaned, re-gapped and reused.