I got one of the older Guide Guns in 45-70. Mine has the ported barrel. I love this little rifle with the big hole. It is used as my black bear rifle and it really 'thumps' them. I don't see why it wouldn't be just as effective on boar.
If I were to get another Marlin, it would probably be a good used 336 in 35 Rem., or an 1894 in .357 or .44 mag.