Well you could download the standard BIOS that the computer would have come with and install it. Of course there is more danger that it will not work and then the computer will no longer be useable. Older computers often have to use the floppy drives to update the BIOS and it is quite possible to run into a faulty floppy drive. The best option is to try to leave the BIOS alone unless there are problems. Too many people assume that changing the BIOS will fix problems. However, if there is no problem dont try to fix it. A search on the internet might tell you if there is a BIOS issue with a specific motherboard or what kind of Processor a specific BIOS will support. Typically a BIOS is updated to support a specific Processor it did not originally support or to remedy technical issues like Memory Stability or support for Integrated Chipset Components.
One issue that you might need help with is aquiring a disk or a download with the needed chipset drivers. You usually only need this if you replace the system Disk (Hard Drive), or plan on wiping the initial Hard Drive wich might be a good idea. I think you said you replaced the hard drive so if you can get a download with the Chipset Drivers, that might help. This usually helps how well all the motherboard components work together. It may imporve how things like Ethernet, Sound, or video work.