Scaling drawings has been discussed on other posts (do a search). .

Basically, lots of detail is lost as the scale gets smaller and member sizes are sometimes bigger than a scale of the original size, mainly due to available material sizes. eg. You would select a slightly thicker plate size in preference to one that is thinner. (you would have to compare the 2 sets of drawings to see if this was actually the case)

Castings should be OK for using either a set of drawings at 3.5" gauge or scaling down 5" gauge drawings to 3.5" gauge. The final model is still 3.5" gauge. But they may not have some of the feaures of a full size loco, depends who made them.

I would recommend re-drawing all of the drawings in CAD. This would check for errors in the set of supplied drawings and allow you to modify slightly to suit sizes of available materials. (some sets of model plans are drawn by what I would call not professional drafting people and often contain errors)

Paul