well... all you naysayers who feel you could do it cheaper, lets hear how long you feel it would take you to prep the boat (and dismantle the cabinetry) so that you could take the original engine out, swap everything necessary over for the installation of a completely different brand, bed it with new motor mounts, in the proper spots so the shaft exits the hull in the correct position, and, dont forget to fill the old mounting holes in the stringers and make the other repairs to them that may be needed...
then, after fitting and removing the engine so the mounts could be drilled and bolted, and the exhaust routed for the new engine, RE-install it all, set it up and align it properly..... then reroute all the existing wiring to match the new engine, with possibly a couple new gauges to match the engine sensors, and then reinstall all the cabinetry to original... dont forget the fuel lines and bleeding them, hooking up the water lines, and while you have the engine out it would only be a fool who would not fully service the shaft/bearings at this time....
and then warranty it......
keep in mind you will be taking all the risks and the customer will return to you, within the warranty period, each time something doesnt seem right (even if everything is fine), so that you can work on it for free....
swapping brands of engines is a very time consuming process that is very much different than removing the engine from your boat, making a repair to it, and then reinstalling it and reconnecting all the wires, cables, tubes and lines to their proper places.
it doesnt seem so unreasonable to me, BUT it is still expensive... this is why old boats with bad engines are pretty much worthless, and why we are driven to be DIY'ers... (I do NOT mean to say the OP's boat is old or worthless, but old boats with bad engines, in general, have little value)
but for those who are not DIY'ers, luckily, there is always someone willing to do the dirty work for them... but always at a price.