All the Duratrax parts should fit. I haven't read any other forums where someone has complained about Duratrax Firehammer MT/Smartech Bigfoot/XTM Grizzly/Carson Comanche/Carson Attack parts not being interchangeable. The only differences are the radios (I don't believe the Grizzly even comes with a radio).
I just purchased a DDM
Pipe for mine based off Bigger The Better's review and video.
A few FG parts are interchangeable:
Link
Towerhobbies has some alloy stuff coming out, easy enough to find on their web site. Seems like most places are out of parts for them right now so hopefully some shipments come in soon. Another place which is in Germany is
HobbyThek. They have all kinds of alloy parts for the Carson Attack/Comanche that will fit the Firehammer and Firehammer MT. The only downside to ordering from them is the amount of shipping you will pay. I ordered a Steel Pinion and a Large Alloy Engine Mount. I spent $130. $40 bucks of that was shipping, but I wanted the parts and no one else in the states had them. The great thing was, I had it here in the states in ONE WEEK. I'm impressed with their efficiency. I've also found a steel pinion elsewhere that I am waiting on. It's from Austria from
Raco. It has only cost me $30 bucks to get it, but I'm awaiting delivery on that one. I expect that one will show in a couple weeks.
So, if you are thinking about a monster truck to kind of mess around with, this is probably the way to go if you want to go on the "cheap" like I did. I wasn't ready to make an investment in an FG. Didn't know if I'd like it or not. My next purchase will probably be the HPI Baja.
I'm hoping someone else will throw in their two cents on the differences so you have a bit more information. I'm a newbie with the big cars but have had about everything else prior to my large scale purchase.
Here is a video collection link:
https://www.largescaleforums.com/forum/showthread.php?t=150
Here is the review and video of the pipe I purchased (Bigger the Better review, I have a feeling he'll have his own RC Review show someday, lookout Chris Chianelli!!

):
https://www.largescaleforums.com/forum/showthread.php?t=420
Let me know how everything ends up!
csp1971