To be fair, many people worked on their car because it was both accessible and a cheap way to increase the value of and/or reduce the cost of a large purchase. Cars' mechanical components have become less accessible over time, and while you can still work on them yourself, it requires either more knowledge to do so or to use more modular components where there's less technical work besides plugging it in.

Computer hardware isn't much better.

Software though, that's where I'm convinced this spirit is more alive than ever. Instead of racing his car your nephew may play racing games, and instead of working on his car to make it faster/better/cooler, he might become interested in making a game, or other software, and the bars of entry for that are lower than they've ever been, and are just getting lower.