Year is irrelevant; Model is everything. If this is an NS40C (single carb, 2-cylinder), it is quite obsolete, but some parts are available. The Factory 1- and 2-cylinder 2-stroke service manual, available from any dealer (including me) is available. 50 psi indicates you may need to hone that cylinder and install new rings. If it's really bad, you may be in for new pistons as well. Pistons and rings are available in both standard and 0.5 mm oversize. Power head gasket sets are also available. If this 27-year-old motor has been running in salt water, it may be difficult to disassemble, due to corrosion of the decades. After you get the head off, you will know more.
