135 across all cylinders is very good, what you really are looking for is a weak cylinder compared to the others.
Try this link
http://boatsetup.com/PlgIndex.pdf
to see if you can identify a pinned head.
There will be what looks like a hex fitting on each cylinder right next to the spark plug fitting in the block.
The pin was added to help deflect atomized fuel as it was being injected into the cylinder.
Indexing the plugs, or orienting them so that the electrode opens toward the injector also helps for more complete fuel burn, thus minimizing carbon. High performance fuel injected racers do this all the time. It takes me about 16 plugs to do a full 12 cylinder plug change to get them to orient correctly, they recommend NOT using spacers.
I would say at that age, if the motors have had regular use you are probably ok.
If you can find a dealer to hook your motors up to a laptop or pda with the diag software for that year, they can tell you a ton, hours at various rpms, code history etc.
Get an Evinrude service manual, not the sealock (sp?) or consumer grade manual and begin to be able to identify alarm codes, how to read them off the dash etc. by holding open the throttle position sensor while turning the ignition key but not cranking will be invaluable, about $65 from an evinrude dealer.
DO NOT run standard TCW3, only XD50
Good luck and please let me know how things turn out or if I can help further.
They should be able to tell you if the upgrade was shipped if you provide them the serial #s
gt