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Figure 2-3 hours labor to get a good look at the problem. Pull the plugs and scope the suspected cylinders. Then drop the oil filter and inspect for grenade frag. If a cyl has failed or the filter is loaded with shiny bits, the motor is now a boat anchor IMO. I would not rebuild a grenaded m96. A clean motor from a dismantler offers a lot better option and higher success rate for my $$.

If everything looks good and clean except the misfire in cyl #4, a valve spring is very likely. This is certainly worth a repair attempt for reasonable $$.