Its black with a speaker shape i doubt second hand would be any good try taking the door panels of off your other doors and have a look i boubt they would all be missing.
Or get shallow 10cm speakers maybe 2 way and they fit in the hole. I have tweeters mounted at the top of the a pillar 10cm(4") mounted in the dash and 16cm in the doors. Pack the dash with insulation (loft type) to get better sound.
Would be easier with the seats out small clip on the front to take out then push the seat all the way back off of the runners then remove rear part of centre console then the carpet should come out fairly easily,
If you dont want to go forced you are limited a shrick inlet mani will help with torque as will a decent set of cams flowed head and if budget allows a 3.0 or 3.1 rebore but it would be cheaper to go forced induction than to have all of that lot done.
Headlamp bulbs as above should be able to be done in situ possibly a cover over the bulb part to remove the foglights can be easier to remove the light from the bumper but can also be done from above in situ.
Schrick inlets do not give 20 bhp when i had mine fitted to my auto it knocked 1 second of off my 1/4 mile time but all cars will be different. They alter the torque curve giving better low down torque iirc.
Im only in Crawley maybe i could pop up with mine so as you can compare im working nights until Thursday then back on days Saturday Sunday so could do Thursday or friday day or evening or Saturday Sunday night.
It's normally the clutch cable on the mk1 that pulls through the bulkhead and to remedy that you could buy a plate for the cable to go through I'm sure it's a different arrangement on the mk3 though.