Did you mean MechAssault?
Anyway i don't think softmods are a good things. First, the latest 1.6 xbox
can't be softmoded that way (needs the UDE thing wich is really unstable).
Then once softmoded, many errors can make the xbox unable to boot. And last
but not least, as original Mech Assault disks are really hard to find
today, the only avaible solution is to install a (even basic) modchip, such
as aladdin (cheapmod with write enabled flash and wich can still boot the
original BIOS when requested).
While it's true that it won't work with a 1.6 Xbox, it's still totally
doable on older models. If you can check before buying an Xbox, you
should be in the clear. As for the resulting errors, I can't speak for
those. I admittedly had a hellish time getting my larger HDD (200 GB of
win), but that was largely due to the software I was using on my PC to
un/lock the hard drives. Mine stopped turning on before I had any
troubles with errors. That was because I loaned it to a stupid friend
who screwed up the power plug connector, though.
As for getting you hands on a copy of MechAssault, you have other
options. I know it will work with a copy of Splinter Cell or 007: Agent
Under Fire (I think, but I could be off on the exact 007 title to use).
At least one of these games, I think the 007 one and most likely more or
all, was discretely rereleased with the security flaw closed, so you
should be careful about getting a newer copy, although I think they were
pretty rare. Regardless of which game you use, they all fall prey to the
same exploit, but it needs to be packaged in a save file for that
specific game. Avoid Platinum Hits versions, those were all fixed, if I
remember properly.
And if you can't find ANY of those three, try eBay or somewhere else
online. None were rare games, so copies should be cheap and easy to
find.