Easiest conversion is putting a 2.0 bottom end on for a bit more torque
Bottom end should set you back no more than £100 but then you will need to budget for a top end gasket set, coolant, oil and filter at the very least
If you wanted to carry on with this engine you are obviously looking at cylinder head options, cams, exhausts manifolds, lightened flywheels
A decent spec 2.0 8v should see you at approx 140bhp
Alternatively for a little more work you could drop an abf in there and have 150bhp out of the box with more potential for upgrades in the future
All depends on how handy you are with a spanner, budget and time constraints really