erm there is no practical upgrade unless your building what you can buy.
if it's a 20vt you chould put a diffrent 20vt in but for cost gains you'd proably be better off forgeing your engine and getting a fancy turbo and a aslippy diff.
you chould put a TFSI in but a forged 20vt is going to make simlar power outputs but admitely use more fuel.
a 2.0 tdi chould go in but thats a lot of work for the gain over a 1.9 PD with tweeks
ermm supercharge a 20vt lump perhapas if you want to be diffrent but thats been done.