Re: Service manual, parts for early 50s Johnson 5.5 HP Sea Horse?
have you checked out duckworks yet?
http://outboard-boat-motor-repair.com/Johnson/default.htm
they do a 5.5 re-build.
Also abos and a few other books have re-build instructions for these motors. I have used them and find them just as handy as the originals. Also the 5.5 spanned several years, while there will be some differences, I've had success using one abos guide for my 54 johnson 5.5, 58 evinrude 5.5 and 59 johnson 5.5.
75 bucks sounds like a lot for a manual, must be an original service manual from 54. have you tried ken cook?
http://www.outboardbooks.com/
also check out the AOMCI site
here's the links page, there are several places to get help, there's also a wanted/forsale list
http://www.aomci.org/links.htm
good luck and have fun, the OMC 5.5s are fun little motors