Some basic parts are still available for the gearcase, such as the seal kit, impeller and maybe a few bearings. Other than that there is really nothing available new unless you can find someone with old stock on the shelf. The 1956/57 35HP should have the same gearcase as yours.
For future reference on parts for your engine there is a company in Seattle Washington that specializes in obsolete and hard to find parts for most marine engines. The name of the company is Sea-Way Marine and their website is http://www.sea-way.com
Some other good sources for used parts are ebay and craigslist.
A quick search on cragslist came up with the following listing
http://tricities.craigslist.org/boa/1378203329.html
A quick search on ebay came up with the following listings
http://cgi.ebay.com/ebaymotors/1956-evinrude-30hp-big-twin-2stroke-outboard-elec-start_W0QQitemZ110439300011QQcmdZViewItemQQptZBoat_Parts_Accessories_Gear?hash=item19b6b1fbab&_trksid=p4506.c0.m245
http://cgi.ebay.com/ebaymotors/1956-Johnson-Evinrude-Outboard-Motor-25hp-Lower-Unit_W0QQitemZ270463269033QQcmdZViewItemQQptZBoat_Parts_Accessories_Gear?hash=item3ef8ddf8a9&_trksid=p4506.c0.m245
http://cgi.ebay.com/ebaymotors/1957-Johnson-Sea-Horse-35-HP-Used-Lower-Leg-Unit_W0QQitemZ330363921740QQcmdZViewItemQQptZBoat_Parts_Accessories_Gear?hash=item4ceb39654c&_trksid=p4506.c0.m245
Obviously with ebay or craigslist you will risk the chance of getting something in as bad or worse condition than what you already have, but this is rarely the case. As far as sea-way, I have dealt them many times in the past and have never had a problem with the parts they have sold. I have a lot of customers with older engines that I have referred to them and have never heard a complaint. I hope this information will help you and let me know if you need any other assistance.
Marine Mechanic
11 years experience working on outboards, inboards, sterndrives & pwcs
Thank XXXX XXXXX for your answer. I don't know how you found the answers so quickly and efficiently. You are good. And , I want to thank you for being so honest about Craigslist. I'll get all my parts from Sea-Way.com.
Thank you
John Tardif
You can call me tomorrow if you like at XXX-XXX-XXXX.
I don't want to put old parts in my new Evinrude. It sets on a 1956 Crestliner 14 ft. aluminum boat. I enter it in antique boat shows. It looks great, you should see it.
Thank you again for what you did for me.