There are several reasons why the X20XE and the later X20XEV produce less power than the earlier C20XE 150ps engine. Basically the earlier engine was designed and produced at a time when emissions regulations were more lenient and as such, the cylinder head, inlet system and cam profiles were all designed to achieve optimum power and torque, and also with motorsport in mind.
As the Euro 2 and Euro 3 emissions laws came into force in the mid to late 1990's GM responded with the new range of "ecotech" engines. These were comprehensively redesigned with more conservative cam profiles, plus more restrictive (but more emissions friendly) cylinder head ports and combustion chambers than the earlier unit. Finally the engines were mapped to run leaner than before, again to help with both emissions and fuel economy requirements.
It has been said that the 136ps is difficult to tune, but that isn't the case. It is quite easy to achieve 158ps from this unit simply by concentrating on rectifying the restrictive areas as highlighted above. Ultimately the Ecotech engine has exactly the same power potential as the old XE engine, as has been proven in both touring car racing and RML's Astra rally car program. It's just that you have to do more to the engine in the first place to remove some of the restrictions.
As for fitting an earlier head to the later engine, they are enough important detail differences to make this an unrealistic proposition - certainly within the confines of a budget for a fast road car. Also given the inherent tuning potential within the motor you'll achieve much better performance returns from concentrating on addressing the areas already outlined.
Should you require more information on extracting more performance from this engine then we would be delighted to supply it.