Re: fix it or buy it
If it truly is just a compression LEAK, the head gasket is probably the cause. Replace it for around $30. If the compression on a cylinder is low(less than 10% when compared to the others), then remove the head and inspect the cylinder for scoring. A new piston and bore job on just that one cylinder is around $400 if you do the work yourself( other than the bore, use a machine shop). The merc. may not be in much better shape due to its age so it is your choice.