An engine compression test is an important diagnostic test that any boat owner can do with equipment that costs about $50. You can also rent a compression tester. The compression test measures the maximum pressure created in each cylinder by the pistons when the engine is cranked. The results of the test can tell you many things about the health of your engine.
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What is the impact on the pistons/rings if you only replace the valves on an older engine? How do you know that you did not also have a piston/ring issue?
The process outlined in this article is incrementally stepped so the least expensive and quickest solutions are first tried. This is the standard way of approaching the white smoke problem.
First, the injectors were replaced and compression test is done to see if the problem is fixed.
Next the head is removed and the valves and head gasket are replaced to see if the problem is fixed.
Last, the engine is removed for a complete rebuild.
Each step can cost ten times the previous one so completing each step, then testing before advancing to the next is the most prudent way to proceed.