You may want to reset the BIOS to the default settings. The setting for "software backlight control" should be DISABLED (the default) and then set the next setting to 100% brightness as I said before.
Resetting the BIOS to the default will get out all the corrupt data from the dead BIOS battery. As the battery discharges, it gets a little weird and corrupt data creeps into the settings. Could be your BIOS battery is not completely dead but under .5v or so. The battery you want is CR1220 available in the US at Radio Shack or Walmart for about $5 or buy a pack of 5 from EBAY for $1. I buy the cheap China pack of 5 for $1 delivered - crazy. I can't use the mail service to send it next door for that price.
With that 64mb flash, you'll have trouble loading anything useful. The DT310 was sold with a 1GB flash as the smallest so someone changed that. The modification to the PATA port is not that hard. I'll see if I can take one apart, get some pics and write it up for all on the forum.
Check this post I wrote about the DT310 for BIOS, batteries, hard drive, etc. It references the "stock" system with XP Embedded but the other info would be useful to you. viewtopic.php?f=42&t=1479#p4759
If you can do the modification for the hard drive then a 10, 20 or 30GB drive with XP Tablet Edition would be the answer for you. The drives are availble as pulls from older IPODs or as new. Search EBAY - lots available.