Some MR11 bulbs are 12V and some are 6V. If you want to keep the same battery you need a 6V MR11 bulb.
Here's a 6V 20W bulb. This should give you more brightness, but will drain your battery twice as quickly as the 10W bulb did.
http://www.batteryspace.com/index.as...OD&ProdID=2248
Just a warning, I bought a couple of these lights a while ago, and I found the light from the 20W bulb about equal to the light from my 10W Trail Rat, and found that I preferred the light from my old 15W Niterider that took 5 D batteries (I used nimh rechargeables making it equivalent to a 6V battery pack) to the light coming from the 20W on this website. You may have a different experience with this.
The LED bulb you mentioned appears to be a 12V replacement, so I don't think it would work with a 6V battery. Also, 80 lumens is very dim. I'm pretty sure a 10W halogen would normally be putting out at least 200 lumens.