The [Flange Focal Distance][1] (FFD) of MD mount is 43.5mm, while the FFD of EF mount is 44mm. That's only 0.5mm for an adapter without optics. For a *hypothetical* adapter with optics, you'd likely be multiplying focal length by 1.4x, so 85mm → 128mm.

The feasibility of modifying the lens depends on its specific construction and design. I'd advise against it unless you got it really cheap, like less than $10. Even if you have a lathe and manage to cut the current EF mount to M42, the FFD of M42 is 45.46mm. There's a good chance the lens would become limited to close-up and macro photography after conversion.

You probably got the Petzval lens for its swirly bokeh. There are other lenses that are known for producing swirly bokeh, such as those in the Helios series. I have also encountered other lenses that produce mild amounts of swirly bokeh that are most noticeable when there are leaves and foliage in the background. You may even already have one without having previously noticed the swirl.

See also:

* https://photo.stackexchange.com/q/62000/75526
* https://photo.stackexchange.com/q/105926/75526


  [1]: https://en.wikipedia.org/wiki/Flange_focal_distance