Well the most obvious that will stop you being able to put it together is that he shows the cylinder and weigh shaft bracket held on by bolts/screws inserted from INSIDE the boiler. Not only can't you get to the inside but if you were tempted to solder them in during boiler building due to their radial positions there is no way you could position the two castings onto them. Boiler barrel and thickening plate should be tapped for bronze bolts or studs.
The shared manifold may not please all boiler inspectors so check first if they want the water gauge fed separately.
He has gone down from 16swg to 1.5mm material but no calculations are shown as to whether this will still produce a safe boiler at the same working pressure.
Stainless steel spacers for the stays will be near impossible to fit and not the right material. Solid stays soldered in are more commonly used now, either rivits or turned from bronze for the threaded ones that take the hornplate fixings
Several other small alterations which make it no longer and "accepted design"
So if you are thinking of using those drawings speak with your likely boiler inspector first before buying any materials
Smokebox from copper may be a bit too soft, better from brass. Don't bend the spokes into a lazy "S" shape like he shows, should be tight minimal radius bends at the of palm and hub.