Redmine

Patch pureftpd-mysql ;)

Seriously, you should consider altering pureftpd-mysql instead of redmine auth-sys: md5 is known to be breakable. But if you want to take that risk (not tested at all):

app/models/user.rb

Replace l.18:
require "digest/sha1"
by
require "digest/md5"

then l.272:
Digest::SHA1.hexdigest(clear_password || "")
by
Digest::MD5.hexdigest(clear_password || "")

this should do the trick, if not, look to user and auth controller, a reference to SHA1 would be easy to locate.
Of course, all your ancient account will be locked out.