Yeah, then your only issue is chainring selection. I have not seen any other off the shelf options to replace that cottered chainring. You could probably take an old cottered crankset and cut off the crank arm leaving the cotter fitting as long as you can get one that will fit the spindle. Something like this:
Then some careful work with a cut-off wheel and grinder and you'd have a smaller ring on a cotter. The one I linked is just an example of what I'm thinking of, not sure of the diameter of the kit crank spindle, I can measure mine tonight if that would help you out.