A 50 hp outboard would be ridiculous on a Grampian 26. Way too much weight in the wrong place, that much HP would just create a gas guzzling wave maker - you would not gain any useful speed. This boat will not plane. Bad idea.... For serious cruising you might want an 8 hp or so just to have a bit more 'punch' in a headwind or chop.. that would be a reasonable thing to do.
As far as the mast goes it may well be repairable, pictures would help to assess the damage. The mast may be in a 'tabernacle' which would allow you to lower the mast with some rigging - certainly makes it easier to address the repairs when the time comes.
1984 Fast/Nicholson 345 "FastForward"
".. 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)