First, what is the condition of the fin? Are there any decals or painted areas on the fins
surface that are not "faired in" with the adjacent foil?
Try some fine wet/dry abrasive paper to "blend in" any raised areas.
The 46 cm Drake fin should be adequate for you 7.5 m2 Hellcat.
A larger fin might work, but a different/better quality fin in the same size
might be better.
Hope this helps,