I removed mine using a lot of force. Pry inward. The drains for the convertable top are behind the scoops. Clean them out, while you are there.
Through the bottom
you just don't ?
Remove the spare tire and the plastic cover in the frunk high and low side are both right there.
The alternator is located by the right side wheel, at the bottom of the timing belt. It is a difficult process to remove and replace. It must come out the top, so depending on the model of MR2, you may have to disconnect the cruise control and move it aside. You can move it out the bottom but you would have to remove the exhaust.
Remove the front bumper to remove the radiator
see this page for pictures and instructions: http://www.padandwheels.com/mr2/alternator/alternator.html
On the second generation MR2 and MR2 Turbo (1991 through 1995 USA, and 1991 through 1999 for the rest of the world), it is on the FRONT side of the engine, between the engine and the rear firewall of the cabin area. It is the right side of the car.
Ask Kody olsand
lift it straight up
right by the front trunk lever on the driver side. it's a small black box hidden under there with a plastic cover that can be a bit tricky to remove.
Its easy open the rear trunk remove the panel towards the engine bay it should be on the passenger side on 85/6 for 87+ it could be in the same location.
The O2 sensor is located on the downpipe of the MR2, although you were not specific to which MR2 generation/model you are referring to. Most are located there. The downpipe will be bolted to the exhaust manifold, which is bolted to the side of the motor closest to the firewall.