Hi Claude,
Depending upon the amount of power you feel you need, the O-470K may be the simple choice. If I recall, the U model is slower turning (to keep the noise down), and does not run on Mogas. It is a compromise, which may not be best for you, as you have the choice. Simple can be best in these cases, if you do not need to change.
Changing to an other engine might require changing other things, including the propeller. I will gather some more information on this, I know exactly who to ask, we have done these changes many times before.
A carburetted 520 or 550 is a great modification, though it is costly, and you'll be using more fuel. Be sure you need that extra power before you consider paying the price for it! The need for a new engine mount will also be a factor, though check yours very carefully as it comes off, I have seen some terribly cracked. You might need to change it anyway. Changing the engine mount can reqiure changes to the firewall and cabin heat system.
I will gather some more information, and provide it to you shortly....