I know it's an old thread but just in case you have not found it yet. A 1 gallon gas tank that looks just like the one the china kits comes with (other than it's alot bigger) can be found at pistonbikes(dot)com for 50 bucks.
I only know that as I have come across one of these MB's on craigslist and started doing some research. I am now convinced I can build a better bike for MUCH less than the guy was advertising thanks to the info I have found on this thread.
After recovering from the holidays I will post up some pics of my first build that I plan to do!