Firstly, welcome to CADTutor andi.
Select everything in modelspace and use the SCALE command to scale everything back up to 1:1 (a scale factor of 1250 in this case)
This is how all objects should be drawn in modelspace, at 1:1. Don't let anyone tell you any different ;-)
Now you can create a viewport in your layout and set the scale to 1:500.