Seems like a lot of money for an engine that may last, but you will have a hard time getting parts for should it last the 30 years that most 400's have run. Generac is a brand name, but one that constantly shifts production and I would not expect to get parts in 10 years let alone 30.
I came across that video awhile ago, thought it was kinda interesting, when it comes time I think I might check into it, just another option ! It may not take that much hp to mow grass esnb74, but some of us use ours for other things , and a bit more power,is an added benefit ! added hp = aded attachments !!