Grant: Like a marriage certificate, we call it vehicle registration card here, it is like your proof of ownership that the babe is yours and yours only with your big name on printed on it.
Unregistered cars: Like an unregistered couple, they are either locally produced or imported, make sure it is not an illegal immigrant (smuggled car), you have to go to registrar of marriage to register with your girlfriend bringing along your birth certificates, ICs, passports etc. So to register a car you need the corresponding legal documents like AP, receipts of duty and excise payments, inspection reports, K3, your IC, passport, driving licence etc etc which I think you shouldn't be bother much as the dealer will guide and handle them all if you buy from them.
How to endorse an engine:
If there were an engine transplanted, get your car to the vehicle inspection centre PUSPAKOM for inspection. Thereafter get all relevant documents to road transport department aka JPJ for endorsement.
Track car can be road legal or illegal. Up to you. Ask the track maniacs here they can give better answer.
Car audio is not a problem i think if you just confined your 20" subwoofer inside your car, not outside. Mufflers are like when you're working out with your babe please make sure tune down your volume or else it is a nuisance to others as well as attract the attention of the authority. Same with your babe' outfit, moderation is important because we are a conservative society, indecent appearance and conduct will likely land you and your babe in jail with fines. Colour change must let JPJ be informed after going through inspection at PUSPAKOM.
Hope this is helpful. I am noob too.