Patrick,
Without frame modification I don't think there is room for any four stroke I know of. The opening in the Worksman frame is snug even for a China girl. I had two frames I have sitting next to each other one day and was surprised to see how much room there is on the straightbar 50 Schwinn Panther compared to the Worksman Newsboy. I had it in my mind that the frames were similar in design, but there's room on the Schwinn for a four stroke (that has been done without any frame modification) where I was amazed at how much smaller the opening was on the Worksman. I was also surprised to discover that the old Panther frame is every bit as well made as the Worksman frame and very comfortable sitting on it... just felt right. A panther with Worksman wheels is superior in my mind to the Worksman. Of course, new Worksman frames can be bought where Panther frames have to be found. The same "board tracker" frame design was used on other Schwinn models and there are a number of old vintage bikes with a similar frame arrangement (Elgin, Colson, Hawthorne to name a few) which I think would lend themselves to a four stroke.
SB