Nice looking car.
Regarding what to do, you should bump up your revv limiter some. Check out how your hp curve keeps increasing up high; well bumping your revv limiter up will continue to make it increase and give you more usable power in the upper range. Someone here has revved a 1.9lTD up to 6500rpm before so the stock range of what the motor can safely do is very conservative. I don't know a whole lot else about this motor, but do focus on the exhaust system, you want long runner "tri Y" style header for best performance, like on a performance tuned naturally aspirated gas engine. Since 2" diameter exhaust piping seems about right on a 1.6lNA, you probably would want about 2.25" diameter piping on a 1.9l NA I'd suppose. Let us know how it turns out!