There's one model seat everybody including Hoye and Ebay have for a reasonable price. But in my experience that seat crowds you too close to the steering wheel - and I'm not very tall. On a tiny Yanmar (my first YM186D) it was a real nuisance, so tight it was difficult to dismount.
Now on my second YM186D, the one I found with a loader, I retained the original Yanmar seat even though it was bare metal. I had to weld some minor cracks, now a couple of years later it has held up fine. I glued in a layer of foam mat plus have a removable cushion on top of that. I like this better than that 'universal' replacement seat, there's considerably more space measured from the steering wheel to the seatback.
I just noticed your photos. Is that seat original? It looks considerably more spacious than that universal replacement seat. If you like it, take it to an auto upholstery shop and get it recovered.