....now it's buggered up.

Today, I pulled my mower out for the first time this year. I changed the oil
and air filter, cleaned the plug, topped it up with new gas, swapped the old
blade off for my dethatching attaachment. She started first pull and ran
BEAUTIFULLY!!!

Great I though... Day was going well. Things were getting done. Got about
two-thirds of my yard dethatched and I hit a 2"-3" rock. OUCH! Mower shaking
around making a knocking noise. NOT GOOD!

I stopped the machine, pulled the plug wire and started checking it out.
Blade & springs seems tight and straight. Shaft seems straight. No loose
bits that I can see. So I start it up again and it's knocking and shaking.
Off it goes quickly.

So I pull off the detatcher and put on the old blade, just to see if I bent
the detatcher. Now she doesn't fire at all. CRAP! Could just be flooded or
maybe the key finally gave way, or the timing it out enough to keep it from
firing.

At this point I'm hoping that all I've done is shear off the "key" and it
will be an easy fix... except I don't know where it's located and I don't
even know if that's the problem. To make matters worse, I don't know the
make of engine. ARGH!

Mower is a 5.5 horsepower Sears "Eager-1" with 20" blade. Takes a Champion
RJ19LM sparkplug.

So... any advise anyone? Where should I start?