That's a pretty flakey reply...
If I were you, I'd go get the parts from him and take them home. Each of your parts will have a part number on them...with these numbers it's possible to get the specs from the manufacturer, probably from their respective websites.
BIOS can be upgraded so I'm not sure just what he's getting at there. Did the machine ever run?
Sempron processors use the same basic archetecture as the Athlons do and will run on just about any motherboard designed to use this framework (I use AMD processors almost exclusively)...
Given the part numbers from the hardware you have, I'd be willing to bet we could find where the problem is and get it running. Alternatively, I can put together a list of components you could purchase and assemble that will run Win98..I just hate to see good parts go to waste.
This is just my opinion, but I think your tech knows there is a problem, but hasn't a clue as to what it is.