I would agree with the 16hp single except for use with the snow blower. Heavy snow can and does require a lot of horse power. If it was me I would go for as many horses as you can get under the hood. I'm now in the process of doing this to a small Kubota diesel tractor because the 48" blower bogs it down. I have a Simplicity 7117 with an 18hp magnum (replacement engine) pulling a 42" blower and at times it is all it can do.
Hope it works out for you.
I think Chris is right, and for more than just the horsepower, with a single the driveshaft may have to be lengthen or customized among other things, the frame on the twins is already cut for a twin and I doubt they changed much on the wiring other than the connector. The Magnum has electronic ignition like the Briggs, so I don't think its going to be a issue. Is the Briggs that's on there a Vanguard? I've never heard of a 19hp Briggs horizontal, but I don't deal with many Briggs engines either. Just get measurements the M18 engine. As someone else mentioned check with the repower sites, but if they spec a Vtwin, expect to pay alot more than a M18 and have to do fabrication to raise the base. Some engines come with the kit for additional $$$