Hi Mansfield 171,
There are a few options that will work on your Carve 171.
#1 The 41 cm "Cleaver" trainer fin will work with your 5.6 and 6.7 m2 rigs pretty well.
#2 You could get a 36-43 cm weed fin that will provide as much area as your OEM stock
Drake 56 cm vertical fin.
There are several very good weed fins in this size range.
Lessacher Duo 36 cm
Tangent Reaper 39 cm
#3 True Ames Stubbie 39 cm.
None of these fins will give you quite the same feel and may not get you on plane as
early, but they all offer lots of area in a shorter/shallower package.