I received the engine in the best condition and as you can see from the attached photos, I installed it and used the rototiller as a cultivator with satisfaction.
The adaptation didn't cause me any particular problems (I'm a retired agricultural machinery worker), but it's achievable by someone with good DIY skills.
I used all the parts you provided: pulley, mounting plate, belt. I even reinstalled the protective cover, reshaping it around the pulley.
I therefore confirm that the products you provided me are perfectly compatible as a replacement adaptation for an 'Iseki' engine on a KS 250 from around 1970.
I used the supplied mounting plate; I attached the motor to it and then placed the assembly on the chassis, aligning it with the transmission pulley, which shifted the assembly slightly to the right. I located the chassis openings from underneath. There, I drilled four 6.5mm holes and tapped them to 8mm to accommodate the (short) studs so they wouldn't hit the underside of the motor housing, and there you have it, the assembly is secured. The belt could be a little shorter.
I more or less (successfully) reshaped the belt cover; because the pulley spacing was shorter, the recessed section wasn't long enough to accommodate the engine pulley. As a result, I was able to salvage the original front bracket, which is welded to the left chassis rail, and by slightly modifying it, I attached it to the corner of the engine plate. This allowed me to reinstall the cover in its original position.
I also shortened the throttle control cable and sheath by 30 cm to improve its response.
After adding the oil and fuel, all that's left to do is go to the garden.
I think it's difficult to make it any simpler.