I have M35B 4 cylinder with 1 1/4" intake. If there is no overheating problem, I would not change anything.

On Whimsea, the intake seacock is 1" dia and connects to the strainer with at least a 1" dia hose.  The outbound of the strainer is then plumbed to the raw water pump with 5/8" dia hose.  The outbound the raw water pump is then plumbed to the heat exchanger with 7/8" dia hose.  Seems to be adequately engineered.

If you have a 1/2" hose barb on your intake seacock, then this "suggests" that the thru-hull is also 1/2".  One way to accurately measure the size of the seacock is to remove the plug in the handle and measure its diameter.

My suggestion is to double-check your measurement.  I'm not aware of chronic M35A overheating caused by a factory-installed undersized raw water cooling components.  I generally look at the heat exchanger as the first place for any overheating issues.

Our M35A intake is a  3/4" thru hull, strainer with 1/2"  ports, 5/8 hose to the Oberdorfer pump. Cooling has been fine unless something gets sucked into the strainer. Have considered the larger Forspar strainer for more surface area to draw from. 
 
 

I had the same overheating problem on Caprice. Our boat are two numbers different so I would thing you have the same installation as Caprice. I looked at everything I can imagine to stop the overheating.  Finally I had a marine mechanic check out the raw water system. He told me to get a new heat Exchanger, which I did and that did the trick. I have no overheating problem even when our sea water temperature goes up over 90 in the summer. At the time I had about 2,000 hours on the engine. HX's do not last forever.