I converted a 9.8 Merc once.
Found a long shaft ES motor of the same year on CL that didn't run and had no title for 100 bucks. If I remember right, I swapped the starter, flywheel, stator, rectifier, starter solenoid/button, and CDI. Some of that may not have been necessary, I just swapped it out of convenience.
By far the cheapest and easiest way to accomplish this is to find an electric start parts motor and rob what you need off of it.
Some motors are simpler than others, on that Merc you could just bolt on the starter and swap the flywheel if you didn't need the charging system. However, on my 25 Johnson, the recoil needs to be removed, and a different bracket setup installed for ES.