I believe you could alter windows.xml like so (untested, top of head answer):
<player_stats_entry backgroundcolor="[darkestGrey]" textcontent="{playerhealthtitle|once}" valuecontent="{playerhealth}/{playermaxhealth}" />
to
<player_stats_entry backgroundcolor="[darkestGrey]" textcontent="{playerhealthtitle|once}" valuecontent="{playerhealth}" />
That should give you a number instead of a percentage.