Any freewheel should fit - your big problem is going to be chainline - I don't know how a single speed freewheel stuck on a hub like that will line up with the chainring on your cranks.
Something else you might also want to try - get a pista axle and some track nuts and swap this for the quick release axle at the back - a bit of juggling of spacers and you'd end up with a nice 120mm spaced single speed hub.
Any freewheel should fit - your big problem is going to be chainline - I don't know how a single speed freewheel stuck on a hub like that will line up with the chainring on your cranks.
Something else you might also want to try - get a pista axle and some track nuts and swap this for the quick release axle at the back - a bit of juggling of spacers and you'd end up with a nice 120mm spaced single speed hub.