The current formula for rebuild is (drumroll)...
HitsToRebuild = ( RebuildBaseHits + (MaxHealth * RebuildHealthMultiplier) ) * PartTypeMultiplier
The "PartTypeMultiplier" is different for each of armor, balloon, guns, and engines. "RebuildBaseHits" and "RebuildHealthMultiplier" are global constants. "Rebuild power" listed for each tool is the number of "hits" that tool contributes for each swing.
I leave determining the value of each of those constants as an exercise for the sufficiently dedicated reader.
As I said before in the note of my post, I was assuming rebuild powers needed to rebuild guns were integers because I could not make measurement beyond the integer level. Making the following assumptions not necessarily accurate to a small degree -
RebuildBaseHits = 9
RebuildHealthMultiplier = 0.05
( Which I have to say describe the equation for the graph I plotted quite accurately considering PartTypeMultiplier of armors have been found to be 1 - transitioning to PartTypeMultiplier values. )
Armor -PartTypeMultiplier = 1
Balloon - PartTypeMultiplier = 2/3
Light Gun -PartTypeMultiplier = 2.315
Heavy Gun -PartTypeMultiplier = 2.3471
( Assuming guns should be 2 and 1/3 since they are close to it, and thirds appear to be in fashion )
Light Engine -PartTypeMultiplier = 1.975
( Assuming light engine should be 2, but you can never know. )
Heavy Engine -PartTypeMultiplier = 1 and 2/3
*Disclaimer*Again, those values were calculated using the help of measurements done in methods not accurate enough to consider reliable.
Waiting for developer approval / correction on them for exact values.