Most B27s, despite what the builders wanted you to think, came with just the engine well, and not an inboard Atomic 4. Ten years ago, on the original Sailnet forum (we Bristolini had 210 members at one time) I did a survey of B24, B26 and B27 outboard owners. (I've owned a B26 and B27.)
I'm presuming, from your question, that you had an inboard and due to cost or necessity, you need an outboard for this summer. Most owners of a Bristol, I think, would dislike the look of an engine hanging off the Alberg stern (though sometimes we don't really have a choice). If you can't recut the engine well inside the lazarette, OK.
Of the 24 respondents, the conclusions were 6-8 HP, 25" shaft, hi-thrust prop, ideally four-stroke. One problem encountered was that the Yamaha and Honda four-strokes couldn't fit inside the lazarette, some just without the engine cover.
Some liked the two stroke Nissans as they were light. I had a two-stroke OMC 25" shaft 9.9, which weighed 90 lbs. One note: If you do recut the engine well, and mount a two-stroke, you have to assure a constant air supply, direct to the engine or it will choke on its own exhaust and keep quitting on you.
From experience, having a pull-start, tiller-mount engine wasn't bad. Not being able to quickly change the gears was harrowing, especially in a harbor. Whatever engine you get, get a high-thrust prop, pitch 6-7. It makes a huge difference, especially in backing ability (usually difficult for a full-keel Bristol).
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if you have other questions.