Series 60 14l engine break switch issue

Recap: there were no problems and you replaced the switch because it was broken. Now you most likely have a short to ground as the fuse blows. The fuse blows with both a new and the old switch.
Safe to say that the switch is not the problem. While possible but extremely unlikely is that something just happened on its own coincidentally when you replaced the switch. More likely is that disturbing the harness moved it to cause the short.
At this point I would try to remove the harness and closely inspect it which may not be easy. Breakers are sold to go in the place of a fuse to assist in finding a short. Short finders are also sold but one from Power Probe would set you back several hundred dollars.
If you decide to take it somewhere for repair I would not go to a general repair shop. Go to one that specializes and only does electrical.