repowering my 20v CC have a 150hp 2stroke want to go much smaller

[willy]

An engine that size will not let you get on plane with the V20, especially with a 4 stroke.
And especially with anything other than an empty lite load hull.
You will use more fuel as has been said but also will use more because you have a hull designed to be efficient at planing speeds, at slow off plane speeds it will be plowing the water.
A good condition 115 hp TWO STROKE!! will be the minimum, they are also very good on fuel. BUT, if you anticipate heavy loads such as people and dive tanks and gear etc. a 150 would be the minimum. IMHO