what you do is get the engine rebuilt and slightly modded, then with a turbo conversion with a decent sized turbo, do a LPG conversion and you should see 350rwhp no worries, with huge ammounts of torque...
my mate had this exact thing done to his z31 300zx (with the Navara engine)
you can find out heaps of info if you do a search, i can send u some links if u want..