Just a note man, if you don't have any prior experience to physical hardware hacking, it wouldn't be a good idea.
For 6 bucks, I would buy one off eBay. I cannot imagine you'll be able to build one for less.
As JRL points out, if all of your connections and voltages aren't exact you run the risk of blowing several ICs on the logic board, in which case you have a much more expensive repair problem than buying a $6 mouse. ADB is 100% incompatible with the serial mouse. While the physical mechanism
may be interchangeable, the similarity ends there. I would discourage any more work in this direction until you have significantly more experience with electronics as this is all starting to sound suspiciously like a similar Classic repair problem you had, which turned out badly as I recall.
As for your original question, the original Mac was designed to be used almost entirely with a mouse and there are some things you simply cannot do without a mouse. The point was to force people to use the brand-spanking-new-heretofore-unknown-pointing-device, rather than rely on the keyboard, which is why there are no arrow keys or numeric keypads on the 128K & 512K.