I asked my partner about this because I have literally no idea so take this with a pinch of salt but this is what he said:
Electrical wiring: The wiring is probably not insulated correctly and is coming in contact with pretty much anything in the engine bay. The engine itself, the alternator, the water pump, the radiator, any of the tubing inside the engine bay. This is dangerous as it could cause a fire.
Agreed
Vehicle structure: Sounds like he has done modifications that have compromised the chassis (skeleton of the van) and has drilled holes through the structural integrity of the van (pretty much anything that isn’t the interior of the van itself like under the floor) This can compromise the structure of the van.
Agreed
Oil leak: Most likely the head gasket has gone which seals the top and bottom of the engine together, this usually happens to older cars and most people get a new car as it's too expensive to replace or the oil pump has failed.
Not Agreed lol
Anything can leak oil, ie the rocker cover gasket, could 1 for lots of reasons, oil pump failing would seize the engine
Tyres fouling: Either the tires are damaged or past the legal limit or they have rubbed against the wheel arch due to excessive loads on the suspension causing the van to sit lower which is what it most likely is, going at high speeds can cause the tires to pop due to the pressure. Lock to lock means when you turn the steering wheel all the way left or right where it can’t turn anymore that’s where the tires are touching in that position.
Not agreed
Over size wheels and have no room to be able do to a lock to lock like the van was made on the standard wheels/tyres.