The outboard version of the Ericson 27 had a well-designed transom cutout for an outboard. There is (or used to be) a participant here that did a 'full Monty' conversion of a solid transom model complete with new solid wood coring. Note that you will probably need an extra long (25") leg OB to make that work well.
AFAIK, other than the 70's E-25 and E-23, the rest of the models were designed for inboards. We would also not want to forget the late 60's E-26-1, with a well for the OB, at the rear of the cockpit.
If looking at an E-27 with a deceased inboard engine nowadays, best just to buy it at a price that will allow you to install a modern small diesel. Either buy one new, or shop around for a good used one that was replaced by a larger diesel.
Plan B is to learn to rebuild that engine that is there now.
I recall an example locally from several years ago where an early Cascade 27 had it's original one cylinder diesel (7 horsepower!) replaced by a new Beta at about 12 hp. Old engine ran OK but the owner wanted more power. The removed Ferrymann engine was sold in a few days for a quick $500.
One other thing to keep in mind is that we buyers were demanding a lot more interior accommodation by the 70's, and all builders responded by raising the freeboard. A talented designer, like Bruce King, could do this and maintain good performance and good looks. Some other (nameless) designs became forever homely, and nowadays show up on Craigslist with the ultimate descriptor "would make great live aboard"...
As transoms got taller, bracket-mount outboards (already disadvantaged by being extended aft over a foot and cavitating easily in waves) became Really Really difficult to use because it was too hard to reach them unless you had arms like an ape.
Sorry to burden you with trivia, but it is a multi-faceted problem.