I copied the picture, lightened it, then zoomed in on it. It is as G Dane states, a crankcase centering pin, used to align the crankcase halves properly before tigntening the retaining bolts.


On that 20hp model (you didn't state the year), there is only one centering pin, located on the starboard (right) side of the block (crankcase) near the top portion of it, right alongside of the top main bearing bolt.


Since the bolts are already tightened, the lack of a centering pin will do no harm. However, keep it on hand just in case you ever need to dismantle the powerhead, in which case the pin would be needed to re-align the crankcase halves.