Well, first: check to see how much the advancer can slide forward or back in timing. A lot of these holes are damaged, as Honda used mild steel on the crank and hardened dowels in the advancer.
So, go to TDC with the engine but removing the #4 sparkplug (and loosening all the others to reduce drag for what you're about to do) and touching the top of the piston through the hole with a cut off piece of stiff coat hanger wire, so you can "feel" it easily. Then, using the big nut on the advancer, turn the engine back & forth a little bit near TDC (BTW, that's "Top Dead Center", if you're not familiar with the acronym) until you're right at TDC. Then make a punch mark on the advancer backplate where TDC actually is: when you're setting timing in the future, you can judge it by the offset between the 2 marks.
Usually, the damage is only a degree or two. This is not very significant on a stock engine, but it does get important if you have high compression pistons or are trying to lean out the fuel mixture to the limit for improved MPG. Since the timing specs are all +/-2 degrees anyway, yours might not be hurt enough to matter?
