Did the Perkins
seize up from just sitting or did it stop with a clank when it was running at some point in the past?
Unless some sort of catastrophic damage has happened it is usually cheaper to fix the original motor
than to replace it unless you are doing all the labor yourself and can borrow larger tools should you need them.
Anyway, without any further knowledge I'd be tempted to pull the injectors on the perkins and squirt some penetrating oil
down the holes, or just any motor oil
you have on hand mixed with a splash or kerosene or mineral spirits and get a large wrench on the crankshaft and see if it will budge after letting sit a while which can be hours or days. If it is still stuck then pull the head
and inspect it, clean up any rust on the cylinder walls you see and put a block of wood on the pistons and whack them with a 5 lb hammer to jar them free.
Your perkins is rated at 45 hp, over 4X your volvo
. I have no doubt the volvo would be awesome for motoring along at hull speed
but I suspect might have a tough time with winds, hopefully someone else will chime in.
Personally, I'd spend some time on your perkins first.