i forgot the name of the website(i think rywire.com), but google search obd0 to obd1 conversion/wiring harness. you will have to plug your old harness into this new one, then the new harness into any obd1 ecu, independently wire vtec, and buy a new 4 wire oxygen sensor and wire it. i did it in about an hour(and im a noob) with some basic hand tools and a soldering iron. the conversion harness will run about $100, as will the new oxygen sensor. the rywire conversion harness comes with detailed instructions and pictures, and their website also has them online.
if you go to mainstream, they can tune using crome(which is free and just as good as the s300 for your setup).
if you're new to this, just ask scotty about doing the conversion for you. shouldnt be much.
obd0 tuning is obsolete nowadays.