Welcome to the Forum John. I have no experience with trolling motors or anything else on a boat that uses 24 volts. First I must assume you have TWO 12 volt battery's in the boat which can be "Tapped' for 24 volts by using the two battery's in
Series. Normally the two batterys would be used in
Paralell for everything else in the boat.
I suspect the circuitry associated with the trolling motor allows this..without subjecting ANY of the rest of the boats appliances to 24 volts....which would blow fuses at the least. So this leads me to believe that the circuitry that you mention ..."that is no longer made", needs to be replaced with somehting else. I suspect a compatible item (different brand) should be available from someone that services bass boats.
There must be a bass boat dealer somewhere around the lake. In the meantime, try this link as a start. They sell bass boats.....among other things. NB
http://www.dougrussell.com/
EDIT: Maybe not. I may be thinking of a place just down the street from Doug Russell which i thought was an annex. I'll have a look and get back.
EDIT 2: Here is the place down the street from Doug Russell.
http://www.boats.com/sites/marineusa-worcester
EDIT 3: Try Google:
"Bass Boat Dealers, Winnipesuakee, NH" I found a bunch.