If you are buying a full Scout, make sure you first listen to it before cannibalizing its platter for use on the 19. You may still like the 19 better. I am curious as to why you would want to buy a Scout with the express purpose of using only its platter and arm instead of the entire thing.
Be careful about replacing the 19’s delrin/lead platter with the Scout’s metal platter. The metal platter will have a very different tonal character which you may or may not like. You MAY gain a little rotational inertia with the Scout platter’s additional 2-3 lbs, but the different tonal character will probably be much more of a concern. The different arm is, of course, another variable. I can’t comment on the Scout’s arm, but I do know that the Rega is a pretty good arm.
If you want to substantially improve the 19 while retaining its general tonal signature (assuming you like it already and just want to improve matters) there are two things that you can do. The 19 MK IV platter is a significant improvement in the area of speed stability and bass solidity while retaining the basic tonal character. Personally, I would start there and try and find one. In my opinion the biggest improvement of all would be an SDS motor controller. Very significant improvement across the board; regardless or platter used.
Good luck.