I suggest you buy a cheap modern digital Transmitter. Two channels are enough, though it is difficult today to find a radio with only 2 channels... They all normally come with 1 Receiver, or you can buy an inexpensive one. Replace the one in your boat. Servos/whinches are compatible with any receiver brand so don't worry about that.
Example:
For battery there are many 6V options. I suggest you get a 5-cell NiMh (5 x 1.2V = 6V) rated about 800-2000 mAh (enough for several hours of sailing)
Example: https://hobbyking.com/es_es/turnigy-...er-series.html

Charger: any generic modern digital charger will do. (any brand - they are all the same inside)
Example: https://hobbyking.com/es_es/turnigy-...e-charger.html

Regards,
Thomas