RE: T-Hawk Battery Replacement?
You definitely cannot go to a 3 cell 11.1volt LiPo with the stock setup, things will smoke [:-] You could go to a 2 cell 7.4 volt LiPo, but you would probably have to get a new ESC that has a correct auo low voltage cutoff for LiPo's. The safest thing to do is just get a nicer brand name battery that matches the volts and cells of the stock batteries, but has a higher mah rating so you get more flying time. That is what I did with my first plane and it made a world of difference. Try batteryspace.com or cheap batterypacks.com. I would also recommend ditching the Tamiya connectors and going with something like a Deans Ultra. Much more reliable.
It is better to just have a charger for LiPo's, but there are several chargers that will do it all, NiCd's, NiMh's, LiPo, Li-Ion.....I think Great Planes and Multiplex both have one. With the LiPo's you need a balancer, it just verifies that all cells have at least 3.2 volts, and then balances all cells so that they have equal voltage.