Hey Diana,
Not sure what you mean about not enough power to use a powered windlass, but a larger battery isn't going to cost that much if they think your battery is underpowered.
I have a 140 amp hour battery that ran a 12 volt polisher for two hours straight and showed no real power loss, so a battery of that size should run any of the smaller powered windlasses on the Muir site. From memory, something like the Vertical Powered Capstan 500 would fit in between the anchor locker and the point of the bow and all you would need to do is make sure the rode and chain fed into the locker correctly. At least that's what the Muir rep at the boat show told me as he'd recalled fitting similar to a Top Hat some time back.
Check the rating on your battery and then check the amperage draw on the powered windlass.
If you want an electrician to help you out I can refer you if you send me a PM