There is a site in the network that centres around motor vehicle repairs: Motor Vehicle Maintenance & Repair
According to their help centre article questions regarding repair and maintenance of motor vehicles is the only thing on topic.
I did a search on their site, but did not find any questions regarding registration of vehicles. This is not true, a bug caused 0 results, but there are actually comparable questions, like this one. That question is closed as off-topic, so it might be a good indicator that it isn't the right site for this type of questions.
Motor Vehicles Stack Exchange is for mechanics and DIY-enthusiast owners of cars, trucks, and motorcycles.
Questions related to the maintenance and repair of motor vehicles and their accessories, internal combustion engines, even in applications other than automotive so long as the question is related to engine or engine accessories.
If your question on registration is regarding which laws there are to follow, your question might be on-topic at Law, but be sure to check their help centre first. When in doubt you could ask on their meta site, if your proposed question would be on topic.