Just a word of caution. Get your neighbor to do the welding, then give him his bribe. You'll get a better job.
On a more serious note I'd also suggest not using the rubber like the photo you posted shows between the engine and mount. That will come back to bite you in the form of broken engine mount fasteners or the mount itself. Bolt it solid to the mount, no rubber or resilient material in there. And yes, the 1" seat post material will be sufficient for that mount. Just make sure the welds are good.
Glad things are working for you.