I look at it this way. This is my upgrade importance hierarchy, you get more expensive as you go down the list.
1. Drivers education. Cheap upfront, expensive if you become addicted.
2. Tires: a good set helps handling (track/autocross) astronomically and can be a reasonable price. Will not directly help power, but you will carry more speed out of corners. To get the most out of them you need...
3. Suspension: sways, springs, shocks and camber plates. Lets you keep the tubber on the road. Essential for the track/autocross if you are serious.
4. Stage 1 weight reduction: you are getting serious so throw out non essential things like the spare, tools, etc, but everything can be put back in if needed. You might even put in light race seats or remove the rear seats.
4. Engine: cams, exhaust, headers, pistons, carbs, bigger rad, light flywheel, you name it. How much power do you want? How much do you want to spend? I saw a race m10 claiming over 200hp. Big buck to do this right and durability will always suffer.
5. Engine v2: turbos/engine swaps etc. custom costs a lot, but dont underestimate the costs of 4. Power is expensive either way.
6. Stage 2 weight reduction: screw this you want a race car. Rip anything out not welded down and replace as much as you can with fiberglass or plastic. Throw in a cage.
7. Realization: it would be much cheaper to buy a modern car to get the same performance. But then where is the fun in that?