Re: Mercury outboard 100hp?
These old Mercs don't use Welch plugs. There may be a portion of the block on the Stbd side which is machined flat and has a serial # on it. Otherwise it'll just be on the serial # plate attached to the transom bracket, staring at you when you're facing the motor clamping thumbscrews.
However, from the pics it would appear you've got a circa-'64 Merc 1000, good ol' motor. The dual-point distributor is a bit tricky to set up but they do run nice when tuned properly.
There's a wealth of info here at the forum, just use the Search feature. Recommend you pick up a service manual if, after you do preliminary checks to see if it's worth working on (such as a compression check), you decide she's worth fixing up.
This site has a wealth of info on Old Mercs, including wiring diagrams, pictures, parts, special tool info, and tuning/repair references:
www.maxrules.com
Here's another site to check out:
www.oldmercs.com
HTH and have fun with your "new" Merc!.............ed