I presume you are talking about Shapeways. You will need to contact the seller and see if they will do an O scale version of the model. Scaling models up or down in 3d cad isn't super difficult usually, but it's not totally simple either, especially as the complexity of the model increases. The 3d cad file will need to be scaled, and have things like wall thicknesses adjusted. There will also need to be a consideration for whether the overall size of the shell will fit within the footprint and volume of the printer, which is quite likely an issue with a full size O scale shell. So it is likely that the 3d cad model will need to be split into parts to facilitate printing. Ultimately, when everything is accounted for and modified accordingly, the model then needs to be uploaded to Shapeways at 1:48 scale.
I don't know what an N scale version costs, but be prepared for an O scale version to be pretty pricey. You'll be paying for the volume of print material for the shell, a good amount of support material for the hollow parts of the shell, plus a factor for the space the model takes up in the printer. Hollow shells can be printed laying on their backs to minimize the amount of support material required, but that orientation will affect the quality of the surface finish of the model. Any surface that is in contact with the support material will have some scarring that will need to be addressed once the support is removed.
The best way to 3d print things like shells is to split them into flat parts comprising sides, roof, and ends, and print them with the finish surface facing up. Of course this will require assembly afterward. But it will minimize cost by minimizing support material and machine space, and improve surface finish quality by minimizing the amount of contact with support for the important parts of the shell that we can see from the outside.
Hope this helps,
Jim
