Zompist has a Sound Change Applier that comes with some example rules:
[sm]//_#
i/j/_V
L/V/_
e//Vr_#
v//V_V
u/o/_#
gn/nh/_
S/Z/V_V
c/i/F_t
c/u/B_t
p//V_t
ii/i/_
e//C_rV
Is there a set of similar rules that would "undo" Grimm's Law sound changes?