Is it legal for someone to fix up old cars in their home garage in Toronto (north York specifically)? Including painting the car, engine etc, essentially trying to bring a dead car back to life.
It could be. You would have to check the specific address here, although not all properties are covered by By-Law 569-2013. This will give you the allowed uses. Picking a random residential-looking location, you might be stopped by §220.127.116.11 of the zoning bylaw, which restricts service shops to being located in non-residential buildings. There are also specific laws about vehicle service shops, which may included a required 3 meter soft landscape perimeter and fence, depending on what is adjacent to the lot.
Check zoning law
What is and is not permitted on a given parcel of land will be spelled out there.
Most “industrial” uses are prohibited in residential zones but restoring a single car as a hobby may be allowed.
Check environmental law
Spray painting is often regulated - usually it must be done in a proper booth. The disposal of hydrocarbons (fuel, oil, grease) is also controlled.
There will almost certainly be noise restrictions in these laws as well as zoning law.
Check work health and safety law
These are often broader than people expect and can cover hobbyists, not just businesses.