Just wondering if anyone knows if there's anything I can do to make these engines rev a bit harder and change the power range as they provide a bit of power and torque low end but seam to die out at 4k? I know Xr heads fit and flow better but it still doesn't change anything just adds about 10bhp woohoo lol, I was thinking turbo as its an easy conversion but I these engines don't like revving

Any advice appreciated