For simplicity the main issues with the M20 engines is breathing (and before all the effin' idiots jump in I said SIMPLICITY), so anything associated with bringing in of cold fresh air and getting rid of nasty hot fumes will gain benefits.
A good exhaust and 6 branch manifold (you will find at least 5 dozen arguements over which 6 branch) will give you over 15 horses, but remember Horse Power is just a number that means little all in the real world ie. tarmac.
The biggest issue with the fresh air is down to the Air Flow Meter (AFM) which restricts the amount of air coming in. This obsticle in both senses will cost a fair bit of money to get around so maybe best to forget about that bit of kit. For reference AFM's were superceded by 'hot wire' systems that do not restrict anywhere near as much airflow.
The M20 is about grunt, well the b25 and b27 (stroked) are, the only b20 I drove made it 20 miles to my garage then got fragged.
So I know very little about them.
Head wise they are all rubbish compared to modern heads (under 10 years old), so personally I would listen to a man who spends his time in overalls
Its worth noting that
1. a good running, well maintained stock engine will be better than a pile of junk with add ons.
2. good brakes and suspension in good order make a car go faster not hp.
Well not for long
Are you located at the ISS? where is it too hard to get stuff when it comes to more power?
