A lot of those Viking 28s had the Vire 7 HP gas engine. Others had a 5 HP Volvo, the one with the flywheel on top. We have three V-28s at our club, and I own a Viking 33. The 28s are very nice racer-cruisers, but you probably know this already.
I would suggest that you can go lower and lighter with a long shaft: a 4-stroke 5 or 6 HP might suffice. Less weight is of course better. If you cruise and don't race, get the lightest 9.9 you can find, and rig a way to freshwater rinse the cooling passages if you sail in salt water.
Another alternative (don't laugh!) is getting rid of the engine entirely, and getting 12-14 foot "sweeps", oars that you can lash to the stanchions. Hey, the Viking is only 4,500 lbs...easy!
I suggest this because I've seen the Viking 28 make 1 knot of speed in 3 knots of wind, and beating everyone else for line honours in mid-summer "dead air" races.