My 0.2 cent:
1. Change ur carburetor to injection
2. Get a suitable turbo (eg Garrett GT25 or Mitsubishi TD04) and all the necessary parts. 2nd hand parts also can, if u can a re-cond turbo also nice, brand new turbo is RM2K+.
3. A very important thing is tuning, get at least something like Apexi SAFC or Greddy E-Manage
4. Standard 1.5 pistons & conrods can boost around 0.3-0.4 bar safely if the setup is right. U can even bring it upto 1 bar if u use hypereutectic pistons & modded conrods, much cheaper than forged pistons/conrods.
How much power u can expect from a turbocharged 1.5? It depends on ur boost & tuning. I'd say expect around 110-120 horsepower from low boost, and 160-180 horsepower for high boost.
Yes, 1.5 turbo has been done oversea and here.