I've owned 3 snow blowers with Tecumseh Snow King engines, including my current 10 hp, 15 year old blower, and never had any problems whatsoever. I also have a smaller displacement Tecumseh on a 20 year old self propelled walk behind mower, which has been OK until last year (carb problem, easily fixed).
I've read that they are particularly sensitive to oil level and oil quality (big end bearings tend to go quickly/catastrophically if starved for oil) so I'm very careful to check oil level before each use.
New old stock OEM parts are easily available through Certified Parts, and small engine dealers.
If the used Craftsman is otherwise clean and what you are looking for, I wouldn't hesitate to pick it up just because it is Tecumseh powered.