Re: 1963 18hp evinrude fasttwin stuck in forward.
Usually, the problem is that it won't shift out of reverse. If that is the issue, take the oval plate off the side of the leg and most likely the shifter linkage has come apart. Remove the TOP bolt completely, slide the shaft back in, then put the bolt back in, being careful not to strip the bolt in the soft brass connector.
If it truely is the stuck in forward, chances are that you pulled the PHILLIPS screw on the bottom of the lower unit to drain the oil, and you disconnected the Y arm on the yoke. The Phillips screw is never to be removed for that reason. You can PROBABLY get it back in right by laying the engine on it's side, removing the phillips screw and messing with the shift lever until you see them line up, then put the screw back in. The screw is the pivot point on the shifter linkage.
If neither of those are the issue, I am stumped! Good luck and let us know what you find.