The PC Engine is great fun to do stuff with, and Mednafen's debugger is also excellent.
While I'm not doing any hacks at the moment, I've done various things over the years (though they've mostly been just making things work, rather than modifying game behavior). Tomaitheous has been much more invovled in serious hacks for the PCE.
While I'm not a huge fan of car-race games, I'm always interested in any hacks etc. which come along.