As a veteran of several "marine" refrigeration replacments, I'd suggest thinking outside the box. Literally. Modifying cabinetry to accept a standard size piece of equipment is a one time operation. It doesn't require maintenance once it's done, and the cost relative to replacing that specialized "marine" refrigeration system multiple times is low.
After having replaced 3 marine 12V fridges in 10 yrs at a cost of nearly $6K, there's a strong case can be made for an apartment fridge that can be purchased at nearly any appliance source that handles domestic fridges.
You'll need to consider how you intend to cruise, domestic equipment will likely use more power than a marine 12VDC unit, but that marine unit may not do as well at the job of keeping stuff cold.
Pay premium prices for "marine" fridge that requires monthly defrosting, won't keep ice cream hard, provides stinky milk in a few days and slimy deli meat in a week. Or get a domestic frost-free that keeps frozen stuff frozen, maintains a box temp of <38F, and won't wilt your lettuce- and run it on your inverter, or fit a dedicated inverter for the fridge. Replacement cost probably <$600 vs up to $3K for "marine". So cabinetry maybe isn't that expensive in the big picture.
Now, all that cynicism aside, you could also opt for conversion of the existing boxes to evaporator plate operation, and oversize the system a bit. I did that for
my holding plate freezer, but rebuilt the box with vacuum panel insulation, a big job, but it works. The biggest consideration is if the existing boxes are adequately insulated, and the insulation is intact- read: DRY. Old freezers have a tendency to develop air ingress into the insulation where the moisture condenses or freezes in the insulation, rendering it wet & non-insulating.
The guiding factor is how you're going to use the boat, and what works best for that use.
