The Shotgun 650 has an 18" front wheel and so that rules it out for me. Current first choice is the Yamaha MT07 AMT, followed by the Ducati Monster.
Less Old Fart & more yahoo! The Yamaha MT-07 is widely considered a "hooligan" bike due to its strong, low-end torque, aggressive power delivery, and playful, wheelie-friendly nature, making it extremely fun!
You've seen the way those guys ride, so it appears that such tyres offer outstanding grip even in monsoon conditions and at 4 or even 5 up on a CG125, impressive load capacity.
In your position, you seem to have decent bit of money owing to you, I'd buy a small place in France, stick two fingers up to the DVLA and this shit hole of a country, and spend all my days eating cheese and drinking wine. Maybe buy a small bike over there for trips.
It is just A THING with me. 17" wheels are just more normal . They are lighter, give the best choice of tyre and lower the bike. It is this pretend "I am an off-roader" thing going on, currently, where you take a perfectly good road bike and make only one modification, an 18" or 19" front wheel in a stupid attempt to fool the gullible into thinking that have a bike that they can take off-road. The Ducati Multistrada and the Ducati scrambler are perfect examples of this bullshit. I have toyed with the idea of ridding the Ridgeway which is a RUPP. Doing it on a Multistrada or Ducati Scrambler would just be insane. I hate bullshit and try to cleanse myself of it, as much as possible. Judging by what hole my compensation money is going in, I fail miserable!.
I bet if you didn’t know whether the front wheel was 17, 18 or even 19” and rode the thing you’d never even notice. It’s a little more complex geometry wise than just sticking a larger front wheel in. Dont let these things put you off, you’ll never know until you try.
My KTM has a 19" front wheel and in real life riding it handles as well as any supernaked. Better, in fact, if the roads are bumpy - though that's down to suspension rather than wheels and tyres. I may convert it to a 17" front with SMpro rims so I can run proper sports tyres as it's more than capable of exploiting the extra grip. But the top end sports stouring tyres that come in 19" inch are so good now that the advantage is questionable. The only reason I'd reject "odd" rim sizes like 18" or 20" would tyre choice limitations. But how much would that matter on a 650 twin making 47 bhp? I wouldn't reject it without riding it first or researching what alternative tyre brands are available in that size if it comes shod in cheap crap. There's a lot of modern retros running 18" fronts these days (Triumph Bonnies eg.) and tyre choice in this size is wider than it used to be. And as Pete1950 says, vintage bikes always ran these sized rims, so if you're building a modern retro it's an authentic look; and if you put 17" on it it might looks wrong, like it was writing cheques it couldn't cash. I'd suspend judgement and take it for a ride.