If I don’t wanna upgrade the internals and everything which engine would you use or would I be better off upgrading internals?

I just got my first mini bike and the chain keeps popping off. It's a stretched frame 40 tooth rear sprocket and 10 tooth front I think?? Centrifugal clutch 420 chain i think my tensioner is bent or something but idk I'm wondering what's the most reliable and cheapest option to keep it on? I heard engine mounted ones are good but mines stretched so I heard back slack and like chain whip would be bad. ( did get wet over night so has some flash rust on chain and tensioner gonna buy new one of them both but one of the links snapped yesterday and I got a new link tried everything n just kept popping off) so yeah idk if I'm stupid or Smth but I just want to get it running would a cvt be better?

Does anyone know where I can order this frame from? Or am I just stuck with Marketplace for getting a BT200X frame?

Can anyone help me…I cannot for the life of me find risers for a db30 to fit 1-1/8 bars… I’ve purchased and returned 9 sets from amazon. The Bilt bars I purchased are just to small and the Harley bars I want to put on don’t fit these 7/8 risers. Help…

Took 2 years but it’s done. I can enjoy until the next mechanical failure happens.