I’ve found spring loaded jockey pulleys effective for flat belt tensioning on drive systems that either lack appropriate adjustably or where re-engineering to fit in less space has necessitated removal of the factory system.

That cast pulley arm looks to be well up to the job of supporting a jockey pulley system. If you engineer the pulley, bearings and spring on a self contained carrier that can be attached to the pulley arm by a couple or four bolts in tapped holes loss of originality will be minimal.

If your useful bits box doesn’t have anything that could be used consider a car auxiliary drive belt tensioner as a source for most parts. On many vehicles the service schedule calls for the tensioner unit to be removed when the auxiliary drive belt is changes so there are plenty about.

The primary issue with VFD boxes when used for machine tool speed variation is loss of torque at low speeds. As such it’s generally advisable to retain the standard mechanical multi-speed drive system. If funds permit a 6 or 8 pole motor is abetter fit than a standard 4 pole one due to the lower running speeds giving a better match of torque to job requirements. Alternatively use a smaller drive pulley on the motor if thats possible. On my last time round that particular loop a second countershaft not only fitted nicely into the proposed layout but could also be done from stuff to hand in the useful bits box(es).

Clive