Chainsaw Effect

It is possible, but the tools are hard coded by assembly code. Unfortunately, for the original game, you can't change it only with the alteration of the data of the Tools items. You need to change the code by hex editing. The relevant codes are:

Pointers for the Tools:

Code:

C2/2B1A: 2A 2B     (Noise Blaster)
C2/2B1C: 2F 2B     (Bio Blaster) (do nothing)
C2/2B1E: 2F 2B     (Flash) (do nothing)
C2/2B20: 30 2B     (Chainsaw)
C2/2B22: 53 2B     (Debilitator)
C2/2B24: 4D 2B     (Drill)
C2/2B26: 57 2B     (Air Anchor)
C2/2B28: 5D 2B     (Autocrossbow)

The Chainsaw code:

Code:

C2/2B30: 20 5A 4B     JSR $4B5A   (0 to 255)
C2/2B33: 29 03        AND #$03
C2/2B35: D0 16        BNE $2B4D   (75% chance)
C2/2B37: A9 08        LDA #$08
C2/2B39: 85 B6        STA $B6     (Animation)
C2/2B3B: 9C A6 11     STZ $11A6   (Battle power)
C2/2B3E: A9 80        LDA #$80
C2/2B40: 0C AA 11     TSB $11AA   (Set death status)
C2/2B43: A9 10        LDA #$10    
C2/2B45: 8D A4 11     STA $11A4   (Set stamina can block)
C2/2B48: A9 02        LDA #$02
C2/2B4A: 0C A2 11     TSB $11A2   (Set miss if death prot.)
C2/2B4D: A9 20        LDA #$20
C2/2B4F: 0C A2 11     TSB $11A2   (Set ignore defense)
C2/2B52: 60           RTS

You can also try the Custom Tools patch. It will replace the Tools attack data with a specific spell data, but you must edit all Tools data and probably update or replace existing spells or attacks data for the use of the new Tools. You can consult the documentation for better details:

http://www.romhacking.net/hacks/2076/