The bracing and the ports should be easy to calculate their displacement as it's basic math. The plate amp on the other hand, I've never used and would hope that they would provide the data in the specifications of the amp the same way subs do. If not, you could calculate it the same way you would calculate the displacement of a rectange/square by estimating how deep it goes into your sub box and that should get you pretty close.
You can use this formula (H x W x D )/1728 to calculate your plate amp and rectangular/square stick bracing (if you're using stick bracing), as long as, your stick bracing isn't wooden dowels. Then you'd use the formula for a cylinder.
BTW, I just found this for you and it should help:http://www.hometheatershack.com/forums/enclosure-volume-calculator/#axzz2HoGJScUIEdited by Audiophile1178 - 1/12/13 at 4:20pm