A 4G91 is NOT a performance engine... if thats what u're wondering.
If u're going for performance... pick another engine.
If you're just planning to swap it into a daily SLOWLY driven car... then this engine is a good choice... the mileage u get out of it is awesome. Assuming no mods are done (even the exhaust left stock standard)... you should be able to attain over 500kms on 40 litres of petrol.
But assuming u wanna transplant and still do wanna do up the engine... ummm... then ah... potentially... this is somewhere around the power of a 1.6 SOHC proton. IF u wanna make it as powerful as 4G93, u'd probably have to spend thousands and thousands of ringgit before u come anywhere close.
4G91 stock as 115hp on flywheel... which gives about 90hp on wheels. GTis have aroud 110-120 on wheels.... to up 20-30hp on a 1.5cc normally aspirated engine... is going to be very difficult. U'd probably need to do port and polish, high cams and an upgraded ECU... which these 3 mods alone would probably set u back at 5-6k... might as well transplant straight a 1.8 in the first place. =Þ
4g91 internals are pretty delicate too... cut long story short... u can't go far with this engine... so if u're in for the power... try starting on a stronger base... like a mivec or a GSR.