I would absolutely go the new plinth route instead of hacking up the original (antique) piece. The new plinth would likely be about the depth (front to rear) of a N33H, just with the Rondine itself mounted forward and a “filler” deck piece at the rear of the plinth to mount the tonearm on the right hand side.. Just did this with a Metzner even though right now I’m mounting up a Weathers regular length arm.. my idea being to be able to mount a longer arm some day.. potentially accommodate both lengths, just one at a time. All the extra deck space to the rear of the table doesn’t look as clunky as it sounds, either. I recently built an oversized plinth for a 1941 Rek O Kut and spaced to allow a regular length Livingston on the right side, then a long Audax upside down on the left side. Works great!
As an aside, I started fixing up old turntables (especially Rek O Kuts) instead of buying and buying better and better “newer” equipment. I find it a lot of fun and a heck of a lot cheaper! I’m old school any way!