What motor size can i use ?

[dave s]

Although not SeaCrafts, my buddy and I have 25' Contenders, and have a similar situation as you.
We both run a single 225 Yamaha four stroke on the original double floatation bracket setup for twin motors; didn't move the brackets.
Mine is a 30" leg and his is 25"; I had to raise my motor as high as it'd go and even added a small jackplate- raised it two inches and the AV plate is still buried. I can raise it even more.
His motor is as low as it'll go but it will cavitate occasionally, usually when it's rough or if he trims up too much.

If you stick with the 20" motor, it'll probably work, but you may want to lower the bracket also.
If you going to repower with a 25" later, then set the bracket higher now.