Ok, I looked around real quick to see who made the Hummer for little tikes but to no avail.
I then decided to look up the motor and kept getting all of this R/C crap. Well, it appears that motor is the same that is used in R/C trucks and cars like the traxxus or whatever. Also the 550 number appears to be related to the power of the motor. It looking at that I a beefier motor to replace that wimpy one that was in there:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=44028&item=5958610 899&rd=1&ssPageName=WDVW
A little later I'll try to see if there are any distributors but I bet your local hobby store has the exact replacement you need.