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easy take the kart as your current car, open the savedata with bruteforce and put the 74 after body on hxd.
after that, copy the savedata to ps3 go into the game and change your current car (kart) to another one then save and now you can edit your kart with (GT5 Garage Editor v1.3.1) there you can swap all what you want (sorry for my bad english xD)

I use the 6 speed transmission from the '98 Mini Cooper code 16AA and the engine tune code 0DCD for 39hp or 0D7F for 46 hp.