I think you meant the drain plug? Cheap fix would be to use plumbers tape.

New ones might not have the quality of an oem used one from japan

OEM oil coolers are made for that purpose and meets the required size, cooling and pressure for it to function flawlessly. My OEM oil cooler is salvaged from Pajero and it's not very big in size, but works great and doesn't block my radiators from getting sufficient cooling. I'm still using my OEM gearbox from the day it left the showroom, still shifts smoothly and gears engage without hesitation everytime. Mine is a pre-facelift 2002 V6 model.

Much higher...in both aspects

it requires about 5 liters of engine oil every 5000kms.

it will also need 4 liters of SP-III ATF every 20k kms and a new ATF filter every 40k kms.

i'm not able to give you the exact cost since price of spares and lubricants as well as labor varies from each mechanic.

but what you get in return is v6 engine power/refinement/mechanical noise/exhaust roar not found in modern 3/4/5 cylinder engines