It might be better not to throw money at the hardtail and sell it to buy something that has full suspension with between 120mm -160mm of front suspension travel.
If this isn't an option, then buy a set of 120mm travel front forks. That isn't said on the basis that I think that you need them, but simply on the basis that you aren't going to be happy with anything less.
Beyond that, the best mod is to fit a dropper seat post.
As an aside, my hardtail copes well enough riding through these woods. Clearly not as well as my full suspension bike, and that copes even less well than an Orange 5 Factory that I sometimes borrow take around there. But it does highlight the terrain that 100mm of travel will cope with at a somewhat reduced speed, although I am never left too far behind.
100mm of travel doesn't seem to cause much of issue with these xc riders either.
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