You know the oil injection system is not working? There is no technical reason to add more oil to the standard 50:1 mix, in oil injected engines that is already a very rich mix until WOT. If the head gasket is shot it will be down on compression on one or more cylinders, it may well run though, just not very happily. Pull the plugs and note if any have water on them - if you get more than a few drops into the cylinder the engine could lock up as water doesn't compress well at all. you need to purchase a workshop manual, and establish 1.) the auto lubrication system has failed. 2.) the compression check is within at least 10%. and 3.) perform a normal major service after the first two to learn more about your engine and to have confidence it is in good order before trusting it on a boat with your friends and family.
If there are issues proven, such as a failed head gasket, you really need to know why it failed, was it corrosion? Automatic lubrication systems are often blamed for a multitude of failures, but on close and skilled examination almost every time it is shown to be a failure somewhere else - sometimes even just really bad maintenance. A pure VRO failure is incredibly rare and using the workshop manual you can prove the system is either working or not. Consider that if you still have oil in the VRO, and you have also added oil to the fuel, you are running up to 25:1 (at WOT) and yes, you would expect that to smoke like a grass trimmer. That suggests to me that the auto oil injection system is working fine at the moment, and you need to fully go through your engine to find any other issues.
Including the water system to know that is pumping correctly and that the engine hasn't been run hot in the past - giving the possibly lazy or less skilled mechanic the excuse that "the VRO is shot".