Running a motor with no housing?

Hey guys, bought a used Craftsman mower in running condition but the blower housing for the motor is missing. It runs really rough after about 20 minutes use and i'm thinking it's due to lack of an air filter? Is this worth fixing or has the fact it's been run without the housing for unknown time likely done irreversible damange? Any advise greatly appreciated.

Running an air cooled engine without the blower housing is exactly the same as running your car without water in the radiator.
I am amazed that it ran for 20 minutes.
Usually they would overheat and sieze way before that.

Yep, you need the blower housing. If it's worthwhile mower, might wanna give it a compression test, before you get too serious.