Saving Old Seagulls

So-Probably a ridiculous question, but I'll ask anyway.
Long story short - I obtained an old Loftus Benner 16 foot Cabin Boat, with a 20hp Mercury installed.
I have my Century as an Auxiliary, but then the Mercury went BANG! and died, before I even got it in the water.
Here's the stupid question - Will my Century work as a Main Motor? I'll be using the Boat in the river, and possibly for toddling round the Estuary, but have NO plans to take it to Sea at ANY point. With a bit of calculation, the Hull Speed is only about 5/6 knots anyway, so would the Seagull be able to do the job?
Feel free to shoot me down in flames...

No worries providing your not in a hurry....

You haven't told us which model of century, the amount of push varies.

Is your water very tidal & how fast is the tide & how windy is your location, how heavy is this boat & how much windage.

The only way to be confident is to try it with the tide running & a bit of wind & with a friend following in a boat that will pull you!