Not highly stressed areas? Paint finish? You could probably do with butt joints. You could reinforce the joints with either dowels or laid-in scraps of wood. You could use back-up blocks, and regain all the strength. Of course, epoxied with structural fillers, epoxy-coated end grain. Since doweling accurately would be an exercise in blue-tinged frustration, drill oversized holes and set the dowels in filled epoxy.
Scarpfs could be cut with a chisel and/or plane. A rabbet plane would be particularly handy since it would enable you to get to the edge/corner. You'd have to back-up the feather edge of the scarpf, on both the boat and the patch, with a temporary block while planing to keep it from collapsing.
With room to work, you could make up a jig to guide a router along the intended incline of the scarph. Additionally, West System's Gougeon brothers would be happy to sell you, or tell you how to make, a setup for cutting scarphs using a circular saw.
I restyled part of the interior of our previously-owned '72 Morgan 27 sailboat. The place was Teak-flavored-Melamine-faced plywood and not stressed. I simply butt-joined the pieces with epoxy. Another place on that boat's interior was the same sort of plywood tabbed to an ever-wet bilge/sole (same thing in this little raceboat). I removed the piece, the settee front, dried it, dug off the rot, coated it in epoxy, tabbed it back in and filled with epoxy and fairing filler. Trouble free. I painted all the Melamine surfaces and varnished the Teak trim so the interior had that Herreschoff look.