Where can I find the mother board for the unit?

Some things aren't repairable after several years of reliable use when wear and tear occurs. However, if you search for used, damaged units, you may be able to find a perfectly good main board but without any guarantees from the seller.

I bought a damaged Jabra bluetooth earpiece for $3 to replace a damaged speaker and/or battery. I rarely use it but it remains functional, holds a charge and still mates to several phones.