You could try get a Honda MC23(Jade)
Rev like mad, but very easy to ride, they are a bit like a scaled down version of the CB1 super 4.
The Honda CBX 250
It is quite old and you may have trouble finding one, but another revvy little beast 24 bhp is not much, until you put it into a machine weighing less than 120Kilos.
Kawasaki Balius, looks pretty and has a sensible riding position, but make sure you put high quality oil in it as it revs to a silly 15000 rpm!
Actually this is a great site for finding any bike sold almost anywhere in the world.
One I have seen is the Yammy SRX 250 which is quite tough for a 250, generally all 250’s get thrashed so I would not look at one with more than 15k miles on the clock.
Maybe the toughest one out there would be the Yammy SR250 but it is rather boring, or the Suzy TU250 which at least has a slightly interesting retro look about it, and it will last at least 30k miles, although in a wintry climate the finish willl not.
If you want bikes that are a bit of fun then I’d take a look for something like
Yamaha Zeal, looks pretty good and is well on the pace for a 250.
Honda CB250RR rare and revvy but capable of 120mph, looks like a miniature Fireblade, rather cool, especially if you can stuff it inside its bigger brother on a twisty lane, whcih is very possible due the the light weight.
Suzuki RGV250 2-stroke screamer, needs lots of love and high quality 2s oil, this is the donor engine for the Aprilia RS250, the latter is actually the better machine.
If you want a 125 then perhaps you might take a look at Cagiva Mito which is a miniature Ducati lookalike or perhaps the Cagiva Planet. Both are 2 strokes, surprisingly quick for 125’s but you do need to warm them up properly before every ride or you can risk ‘cold seizure’.
I vagually recall that way back in the 70’s Ducati had a couple of single cylinder things but I would be surprised if you can find one in good order, much more a collector machine nowadays, and of course there are the legendary Yammy aircooled RD 125 and RD 250 or the RD125LC and better yet the RD250LC, hooligan machines that demanded you ride with at least 8000 revs on them at all times…those were the days