I think your Nirve is steel and if you feel lucky, maybe you could spread the rear ....15mm is like 5/8" for a rear hub motor. The rears have a nice curve on your frame. It might go well w/o stressing the welds, but what do I know. My steel Trek 800 was 130mm, so I only had to spread it 5mm. Used a scissors jack and didn't even think there were any implications.
You probably want a mondo uber motor though, and you would also lose any remaining braking power that you bike has.
Consider a mid drive. Maybe it can be mounted inside the frame, rather than in front. Except you have to get one that works with coaster brakes. They make a version of the TDS-Z2 for that.
How did it go with your Trek? I’m thinking this might be next years upgrade but why not think ahead. I am going to inquire around with LBS and see if there is any interest. I am also thinking about putting the same motor in the rear to uncomplicate matters and have everything run off the same throttle and pedelec. She will be a Direct Drive monster at roughly 2500 Watts. Also dual regen should be plenty of stopping power except below 5 MPH... where I will have nothing LOL . I still have not worked that part out yet, but I’m not close to actually doing it. I will probably have some mounts welded on for at least some caliper brakes to stop below 5 mph. All still in the early planning stages. I am going to ride the hell out of this bike as it is all this summer and get used to her. Still very busy continuing to customize what I have already done. Thanks for the helpful reply.