This has been done numerous times.. probably most often on boats originally equipped with the OMC saildrive gas units, which are notoriously difficult to keep healthy.
Aesthetics aside, the outboard is going to be weight where it's doing the least good. However there's no point in dragging the deadweight of the old engine and drivetrain around - that space can be put to better use. The other issue is removing the drag of the saildrive/shaft&strut and structurally filling in those holes.
On most boats I don't think the weight redistribution is going to be worrisome, esp if you're not trying to race competitively. If you must go this route, then cleaning up the old and doing a proper job of mounting the outboard mount on the transom are of more concern, along with the limitations of a transom mounted outboard vs the inboard installation.
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"