Because you said Donor, as in something as a basis for a build....
An ariel square 4.
Maybe as an a visual excercise - as an engine they were crap ! Very heavy and very slow. Torquier than yer average twin which is why 90% ended up lugging child adult sidecars !
The Healey Bros. [ 20 minutes from where I live ] built a limited edition range of s/arm bikes based around the Squarriel lump - looked the biz but were outperformed by Japanese 125s ! Around 1972 - 78 I think .. didn't last long though. Tim Healey was the driving force and the basis was a modified Egli frame.
I remember one being used in a Sprinter [ Drag bike] back in the late 60's. Shorrock supercharged and running Methanol. It exploded big time on the start line scattering engine bits for hundreds of yards around. Sounded awesome through 4 open pipes though !
I had a pal [ Steve Wassell ] who's Father founded the Wassell spares empire in the UK. They had a really neat private museum and one of the bikes was a factory prototype Squarriel in a swingarm frame [Ariel ] - too late to make it into production and save the factory.
They had a knack of siezing the right rear cylinder due to poor cooling. The bike shown is the four port - preceeded by a twin port, which was even worse.