I have an old skylark 56, I think it is a goldberg model. I no longer have any of the instructions that came with the plane. I need information about engine size and how to set the engine offset. I also need information on the position of the center of gravity after this its flying time

You lucky dog! That airplane is a classic.

Any 40 - 46 size engine will do fine. engine can be anywhere from zero-zero to 2 degrees right and down.

Start with it balancing about 1/3 back from the wings leading edge, and adjust as needed (unless someone out there has the actual plans showing the exact CG).

I agree with Mike's comments.
I used to have a Skylark and flew it on an OS 45FSR which was ample for it.
I flew that plane for 100s of flights and finally retired it because of some fluel soaking in the balsa all over the engine area.

This was one of the best flying planes of it's era.
From memory, the thrust was set for 2-2, that is 2 degrees down and 2 right.
Start with CG at the spar (most Goldberg designs were like that) and you will be fine.

Happy flying