Right now on eBay is a crank mechanism advertised for the 28-29 and 30 Plymouth. It is a little different from my 30 (it has inclosed gear box) but should work well for any of them.
Listing ends Mar 28th at 13:12:14.
Good luck in your quest.
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=220758290503&viewitem=&sspagename=STRK%3AMEWAX%3AIT
Gary