pckeen, based on the serial number starting with a 7 means it is a Canadian mode. That was the easy part.
for 20HP, 80/81 had a starting sn 7148238. No entry in any of the tables until 1986 when the sn start with an alpha character (A708966)
BUT, both 18 and 25 hp motors pass thru your sn numbers.
for both 18HP and 25HP, transom sn of 7183895 would indicate 1982. The motor sn 7202509 would indicate 1983
Can you determine for sure what HP you have or is this close enough.
Fun Times - can you help out here?