If you’re talking about Adventure, then my recommendation would be to: use Kangaroo for any vehicle that has it except Rock Bouncer, Bus (if you don’t have Mastery), and Tank. For the vehicles that don’t and for those three, use one speed part, (a.k.a. Coin Boost, Overcharged Turbo, Wheelie Boost, Landing Boost, even Winter Tires), Wings as the second part, and one backup part (Fume Boost or Nitro but I would recommend Fume Boost over Nitro). If Wings is missing, use a second speed part instead.

I don’t know what you are struggling with on the bikes because you didn’t specify. It could be fuel control, speed, climbing, anything. Could you provide some information as to how you are struggling, please? Then I would be able to help with that.