Handlebars I’ve always used Pro Taper for MX and Renthal for trials and road applications, both suitable and made for all applications.
I agree with the above, with suspension set up correctly you’ll gain overall speed, I’m not a sports ATV guy but brakes haven’t been mentioned yet, not sure how the stock items hold up when going quicker?As slammed said, making it wider with better suspension will make the biggest difference. I feel like the right setup in those areas is more important than having a hot motor.
Rob