Assuming the engine is going to be exposed to the elements, an enclosed type seems wise, as well as being advantageous in the cost department. The magneto would need some protection, and probably the carb.
I would have thought a small engine of the 1-2 hp size might lack visual impact, so maybe something of the 3hp size or so would be visible behind the petunias and be more striking, while not being too large.
Architectural - hmm. The Bamford verticals spring to mind, the OV in particular, with spoked twin flywheels, and that rocker cover that looks like the cast top of one of those ?Victorian water drinkers . The EV and EG have a tapered monolithic look about them which has always struck me as Art Deco in style. If you do use a hopper top, you might find garden birds drown in it, so some sort of mesh over the top might prevent this. The Bamford verticals have the advantage over the Lister A and B types of having enclosed valve gear, which would be good news if they're outside. Of course, if you want to be a bit obsessive the engine could run a centrifugal pump to put some pressure into the water feature! Direct drive would avoid your belt rotting and slipping.
Another idea - what about a Lister D Pump Set with a H2 pump or similar on a cast bedplate? It would drive the water feature nicely. You could put it on a plinth if the delphiniums obscure it.
Of course if you want to avoid the magneto problem a lamp-start could be used, but more quiddage would be required.
Is it going to be in a roofed enclosure?
The best engine garden ornament I've seen is that big Blackstone at the Anson, a good runner, despite being coated with algae.
Think: "Now if I made a roofed shelter and put the big Petter S under it....., and got that pump and dug a pond, and ........................."
Quite the most aggressive case of Petteritis I've seen :-) However let us not lose sight of George's question..... A diesel would address the worries about mags and carbs. How about a Blackstone DB which is totally enclosed and a rather pleasing shape? Perhaps under an arbor of roses (Rosa Alba naturally :-) regards Roland PS In an area of horizontal rain a roof ain't much use Arthur :-)
Hi George, I don't know how much room you have for this but I have seen an Allen Scythe used in an ornamental manner. I don't know if it ran but it looked quite good. I must admit that I prefer to see machinery doing a useful job of work rather than just running unattached. The idea of a pump running a fountain appeals. Of course the idea of sitting listening to the calming effect of trickling water rather changes when it's drowned (no pun meant) out by a loud thumping engine. The reliability of diesels over petrol would help maintain the startability. How about a useful engine with it's own cover. An old tractor perhaps. Working Fergie 20's can be had for relatively few £'s and can be used as well. Smaller versions called vintage sit and ride mowers are also available. If you have a little more space, I happen to know where there is a vintage digger that the owner may be prepared to part with. Come to think of it, all that rather describes my garden. Wifey thinks I have a few too many "ornaments".