Most likely the bat is weak because things are gummed up, or the linkage is worn out.
Before you start to change out the coil, take everything apart and clean. Also, possibly put a new sleeve in the coil.
As a last resort, you can take a few windings off your current coil to get more power. I personally would not go crazy and take too many off though.
But, honestly, do the first part and clean and tighten up the mechanics and you probably will be good