Seeking prop for old Mercury outboard

[RLymburner]

We have a ~50 year old 50hp Mercury outboard on a 14' Glastron at our family camp on a lake in Maine. Prop fell off and the replacement we found laying around just isn't cutting it. Any idea how to spec out and find a cheap, suitable replacement? Primary usage is tugging kids around behind us on kneeboards and tooling around the lake.

Any suggestions? (other than getting a new boat!)