The Ninja 250 is the same thing as the 650, twin parallel up seated and conventionally sprung. Should not even carry the tag ninja imo, sorry if that offends my friends Mordeth and MJB, sure they are great bikes for what they are but they are not Ninjas they are to the Ninja what the SV is to the GSXR.
What the cops look for is the sportsbike fairing they see a fairing and they’re interested, easy to distinguish the legal 150’s from anything bigger.
You are thinking about the 2 stroke 250 sportsbikes but thats not a practical route either, they are old with parts hard to find and very loud so the rozzers will clock you very quick with your large fairing and double 2T pipe to boot. They are more expensive than they should be because good examples are rare and due to age and the nature of 2 stroke engines will likely give you trouble add to that the fact that you will have trouble finding reliable mechanics.
Next you may likely consider the 400’s which, though not as expensive have the same troubles of age and difficult to get parts, include the fact that it may be a cobbled together job from various bikes stolen in Japan as well as having borrowed parts to keep it going and not legal as well as being easily recognisable and in demand by police with their nifty fairings standing out among the sea of scooters.
Hell I have even ridden 2 stroke 250 trail and motox bikes on Taipei’s streets without the cops caring as do many guys I know, but the word is stay clear of the fully faired illegal bikes.
What you want is a great deal that comes from an old bike and plenty of them, like the UK, well thats gonna take a few years if ever since they only became legal recently and even then there is not a large supply because the original uptake was so low so you will never get the deals from the UK here even when the bikes get older because demand here is high relative to the small supply. Do daily searches on the internet and notice how bikes are either snapped up in a few days or they linger for months even years, there are few good deals and many willing to snap them up and the crappy deals are what is left.
I have not gotten concrete answers on the consequences of being caught with an illegal bike on the road beside the fact that you are guaranteed to lose the bike on the spot, but I have heard rumours of deportation, large fines etc.
I have been down this road for the past 3 years with a fine toothcomb and have resigned myself to the reality of the situation, If however you find a better solution than mine please share I would be happy to learn of it.