what size shaft length?
 

I'm keeping my eye out for a replacement motor for my 20. Am I looking for a 25" shaft? Thanks.

Not sure what rig you are running, but recommendation to me from others on this site when running a bracket/platform has been to run a 25" shaft.

I have a 25" shaft on my 96 Johsnon 150, 1987 V20.

I'm running off of the transom on my current boat...

You are looking to get a bracket on a full transom. You can choose whatever motor shaft length you want.
Wowever, a 25" will allow you a lot of play room as to not drowning the powerhead during normal operations.
When you get the bracket and motor, ask these guys here that have done a bracket installation and they will give you some measurements to install the stuff at.
25"!!

Sorry for the mix up but right now i'm running a normal transom mount. Was looking at a boat with a sea drive, and am considering another but it already has a bracket on it. Right now though it's a straight transom mount. Thanks again... Is 25" the way to go for a transom mount?

well if your boat is 20 inch and you have a good running motor run it till it dies, but if you get a deal on a 25 inch motor then just find a manual jack plate used and good to go.

Gonna run this one till it croaks! Hoping that's a few years...