I think you will find all the information you require in the manual at
http://boatinfo.no/lib/evinrude/manuals/1971-1989johnsonevinrude.html#/0
Its important to get the number of turns to prewind the recoil spring right when reassembling ........ looks like its all the same as my 6 hp in which case iits 12? turns... but check it should be in the manual.
refitting the recoil probably the worst part but its a fiddle rather than rocket science.
Less than an hour I'd think........ only a few minutes if you ever have to do it a second time probably
Broken that little plastic loop I bet?
(Later models can be fixed easily because the top comes off the carb, c/w cam follower, without needing to remove the recoil)