I thought most/all Japanese Nintendo cart-based games weren't sealed in the first place? So buying a "new & sealed" one is a bit of a gamble. I often see retro games being sold as "new" (新品) on Y!JA and the ending prices are many times more than the perfectly acceptable mint copy. How can one know if a game is truly new and unused? One only has the seller's word, and not the AFAIK non-existent shrinkwrap as proof. What's the difference between a 新品 (new good) and 美品 (mint) Super Famicom game anyway?
In any case, yes, it's probably easier to replace the tray with one from an el cheapo game. On the subject of replacements/mix 'n' matchin', I would also go as far as to replace the front/back of game carts (special screws required) if I think it has too much yellowing or any yellowing at all. The exception would be "Revision ROMs", if you're anal like me - in these cases, I just leave the back as-is, for identification purposes (i.e. the letter imprints on the back label).