From page 10 of the MN Boaters Guide 2010
COLLECTOR WATERCRAFT
Watercraft that were built before July 1, 1959,
and are used solely as a collector?s item, have the
option of displaying the boat registration number
and current decal on the forward half of the boat
on both sides on a detachable plate or board.
The number size, type and decal placement is
the same as noted on pages 3-8 of this guide.
The "Fees for Watercraft Registration and Titling" chart on pages 4 and 5 do not separate 'classic boats'.
It would appear that the registration fee is the same for all watercraft.
http://files.dnr.state.mn.us/rlp/regulations/boatwater/boatingguide.pdf