On normal tracks, you can use Autocross objects to modify the already existing tracks (so like using ramps on the track or having checkpoints in a different location). However, remember that AI drivers will use their normal racing line, so you can't use this method on any tracks where AIs can't have a racing line (i.e. car parks, open configurations) and you can't block the normal racing line much - otherwise they can't finish their lap!
And, as of 0.7G, you can also control AI in pretty basic level using InSim: https://www.lfs.net/forum/thread/110309 - however, according to Scawen, that is not intended to racing, only for cruising and training. But technically, you can create a custom layout on Layout Square, Autocross etc. and then use InSim to control that AI to drive on that custom track/layout.