Trying to look at this systematically -- basically -- a substantial proportion of the responses here, are helping you gain information about the condition of the engine.
This data (oil filter internal inspection, sump pan with screens inspection, oil pump inspection, oil tank / lines / return valve inspection, valve cover and cams inspection, oil pressure circuit check, etc.), once collected, can feed an assessment of what next. If the data suggests clean and go, great. If the data suggest open the engine, then the better part of wisdom is a replacement, as suggested.
Of course, some people like to tinker, and maybe you're one of those. Like the person who builds their own airplane, the FAA says a priori that person is qualified to work on it even without an A&P certification, you might enjoy the process of learning first hand how Triumph engineered and manufactured that motor, and having pulled it apart, and spent the money on needed parts, you could end up with a "zero miles" motor.
And the possibilities are infinite. If you go that route, you could consider substituting some parts for a little or a lot of boost in performance. "There is no replacement for displacement" goes the saying around the race track, and this certainly applies to the Rocket.
If time is not on your side, as someone suggested, you could part out the bike, easily making your money back, and instead, shop for an excellent used bike all ready to go, with everything working, and probably some added farkles as well.