Mercury 4 to 20 hp powerhead swap

[kaifeiji]

Like everyone, I'm looking for more power without spending lots of money. I just purchased a 4 hp 1972 Mercury outboard and was thinking about swapping in a bigger powerhead. Does anyone know what might bolt up? I found a 7.5, a 10 and a 20 hp head for good prices but I don't want to purchase something that I can't use. I know that the lower units for those motors are different; i.e., they have a reverse gear, more gear lube capacity, etc., but if the bolt patterns, water passages, exhaust ports, and drive shaft from my 4 hp unit line up, there's no sense in not installing a larger powerhead.
I've searched for hours on the internet trying to find out what might interchange but for some reason, not many people mess with these smaller motors. I was hoping some of you might have a bit more info. Thanks in advance for your help.

[kaifeiji]

I thought that might be a tough question. I'm going to keep looking for the cheapest powerhead that I can find and when the engine stops running properly, I'll try the swap.
For now, I'd like to find a 20-25mm Mikuni carb, make an adapter and bolt it up to the block. I also going to attempt to fabricate a tuned pipe, (similar to dirtbikes), and see if I can alter the powerband to fit the carb. Hopefully, those mods should result in a slight increase in hp. If anyone's interested, this website discusses a few aspects of building a tuned pipe:
http://www.hpt-sport.com/tunedpip.htm
I'll keep everyone posted on how this goes, along with the KG-7 rebuild.