Inspired by a friend of mine who was rebuilding the engine on an old car of his, I decided to buy a 1990 Mazda Miata to tinker with during my senior year of high school.
After owning the car for a couple of months, it began to run poorly. I performed a variety of compression tests and found that the cylinders all had low compression from worn piston rings. I decided to tear apart the entire engine and rebuild it despite knowing nothing about engines.
Over the course of 3 months, I rebuilt the entire engine with just a few simple tools (socket set, screw drivers, pliers, torque wrench), a cheap engine stand and a borrowed engine lift. I used online forums, miata specific books and youtube to help rebuild the engine properly. To fund the replacement parts completely, I picked up free car parts from craigslist and resold them online for profit.
After rebuilding the engine, I tested its reliability by racing it at a local autocross event. I was able to sell the car under a year after purchasing it making 4x profits from the initial purchase price.