Sorry Dave, I'd forgotten which engine you were talking about, and thought you were referring to a white Briggs, which would be a classic from the 1970s (black ones tend to be 1980s, and therefore relatively undistinguished).
Tecumseh engines are not actually popular, their reputation was not good and the company went to its reward about half a dozen years ago. However the engines will eventually be collectible, and their unpopularity will make them rarer and more sought after. Think of the 1940s and 1950s British engines: an awesomely crummy load of unpleasant material, but avidly collected these days.
If you look forward 50 years, your white Tecumseh may be as sought after as a British engine of similar age. If you don't want to wait that long, paint it purple if you want to, it's your engine, and currently of very little collector interest. I don't think anyone would be accusing you of vandalism in the next couple of decades at least. After that perhaps they will, but you may not care.
