Have you ever seen the engine mounts in a modern diesel car. They huge and not only that they also electronically controlled to smooth out the nvh. this is hard to over come in an aircraft when its fitted with a prop.
And as I recall now I may been wrong but don't they fire via compression, hence the rattle. Now the rattle is the problem as it sets up the harmonics though the whole of the airframe, as I said earlier a TB 10 or 20 was fitted with one and it blew the tail off it as it taxyed back after its first and only test fight, then you have the problem of a prop. Now as I understand they also fitted with a cushion rubber drive to take the pluses out.
In a modern car this is not so much as a problem as weight is not such a primary concern but in an aircraft it is.
Also as I said earlier im not sure you be able to do a prop balance on the airframe with one of these engines,
Cheers