Sounds like the improvements could cost more than the acordion. :)

1. Replace all of the springs with stronger ones.

2. (I was going to say replace the reeds)...but try replacing all of the reed leathers to improve reed response.

3. Look for leaks everywhere and fix them.
-bellows frame, bellows, "flapper" valves and valve leather.

4. Adjust every single reed one by one for best response and volume. (retuning will be required after this)

DP