I have owned three Chinese bikes, A Mash 400 scrambler, a Zontes T310 and presently a Zontes M310 Scooter. The mash engine is a copy of the Honda xr400 it was a good bike the engine was very good, easy to do your own maintenance, around the bike yer there was places you needed to keep on top of but for a bike that cost just under 4000 at the time what do you expect, I changed the spark plug for a different heat range and changed the battery (Most of the batteries on Chinese bikes are not the best) and consider how long the bike had sat around more so because of Covid at the time and room on ships was being prioritised for medicines. I had no problems with the bike at all.
The Zontes T310 had STAINLEES STEEL SPOKES on the outside to accommodate tubeless tyres. LOTS of stainless steel bolts, an electric two position screen, switch gear that LIT UP at night, a stainless steel exhaust system and a tail pipe that would shame anything Honda has put on its worn to death 500 range! And a Gel seat. Oh and a colour tft screen with fuel trip tyre pressure and two riding modes and different screen layouts. It rid well handled well with a top speed of about 90mph.
Yep, every since the XR400 I think that had been an ideal capacity as a single or twin. Not as vibey as a 500cc+; more poke than a 250. That engine looks tiny: KTM must have had a hand in it.
Must say I never saw the GP450 as a contender despite some interesting tech: wrong engine and loads of issues, iirc.