Yeah the 1.9 and 1.6 gti's used the xu block psa engine which i stronger than the tu, so a 1.9 would be a nice swap into the 306,
1.9 has the same bore as the 1.6 but a longer stroke....the reason why it doesn't rev as freely as the mi16 or the gti-6
still a good engine though, you can use the gearbox you got now on it if you want more accel but sacrificing top speed.... to be honest should yo be doing above 100 mph?,
Get some bigger front brakes (306 gti-6 or 206 gti180/307hdi/307 2.0 sport the last 3 are the same)
and I would recommend you fit a rear beam with the disc brakes they all share the same stiffer rear anti roll bar as the gti6 so it doesn't matter if you get one of a hdi or xsi it will be the same, also the phase 2 and 3 ones have slightly bigger rear discs
is your 1.6 a black top or silver top by the way?