Classic's pm lmit can be edited through a code edit, i have no clue where it is, and i don't have a working install anymore (in fact, most of us have migrated from classic since it's reached its end of life)
You don't need a running Classic forum, just the files.
This looks like it:
In /cgi-bin/
ubb_pm.cgiAt or near line 248 find:
if ($index_count > 59) {
&StandardHTML("$vars_wordlets_err{pm_user_max}");
Change the 59 to a higher number.
