So doing a lot of thinking on a new setup, I will probably be movin to a 16x38 and have this question. I have a 20hp longtail, if I wanted to get more power for this boat should I buy a new 23 and replace it, sell the engine/frame together and buy a new one, try to do light mods, or nothing as it won't be that noticeable. Why?

I would buy a new 23 complete with frame and sell the motor you have. That way you have a motor ready to run properly set-up and you really didn't spend that much.

Your 20hp is a honda I presume, if so, the power is comparable to the 23. This is because the way honda and Briggs rate there motors is different. The Honda is much heavier though and parts are more expensive.

Yea, your right. Not much of a upgrade in power if any but the 23 can be tweaked real easy. A little porting on stock heads makes the low end come to life.

It's actually a 20hp vanguard

Would I e out more money buying a replacement engine an selling my engine or buyin a whole new motor (frame an engine) and sellin my old one as a whole?

Us there any basic mods you can do this block/20hp? There doesn't appear to be anything bolt on out there.
Would a 20 run well on a 1638 ?

Just depends on your hunting load

I'd say typically I hunt alone or sometimes with 1 other person and take a dog(small/boykin) and a dozen decoys. I've been getting by with a 1436. I have tried to take out 1 other person a dog and a big spread for dovers before and the boat/engine did not like that.