So if the car mod that you want to install comes ONLY as a replace, you wont have the handling.meta and vehicle.metas. Most likely I would think the author would swap the car with something comparable to power, spec, car type, etc. But to answer your question tho, there wont be a customized handling.meta to replace. It will handle just like the car you replaced. For this instance, I recommend changing the <HANDLINGID> in the vehicle.meta to something you think would match.
So for example: I simple change the way the NINEF handles by changing it to SULTAN.
If the car comes as a addon, but you want to replace instead. You simply add the car mod file into your dlcpacks, and extract all the files you need. Then you pick a car you want to replace it with and start changing all the files. GOod luck man!