Hi Al - like Brad, I've never heard of anyone making spare parts for Mignots. However, I do know of two UK makers of Napoleonics who might be able to help.
Have a look at the online Napoleonic (54mm) ranges of Irregular Miniatures. They make a number of castings - and arms, heads, weapons etc - might be suitable for adaptation to fit your broken Mignot? They also do a 42mm range of figures which look very like Heyde - with arms/heads/weapons to suit.
Another supplier of spare parts which MIGHT suit - is Dorset Soldiers ( Giles Brown).
Maybe send either of these two, a picture of your broken figure - and ask if they have spare parts which might fit ( or made to fit - with a bit of coaxing).
I have found BOTH of these suppliers VERY helpful, in the past - and whilst your repaired figure might not look EXACTLY the same as it did originally, they can be made to look very similar, with a bit of care.
I always think that ANY repair of a broken figure, is better than leaving them as broken figures - so maybe have a look online at their ranges - and then give one (or both) a try???
Best of luck - Johnnybach