What about the Pirelli Cinturato?

https://www.longstonetyres.co.uk/classic-car-tyres/morgan/04-apr.html

I beleive that Morgan did fit them as standard at one point? But importantly a Cinturato will handle better than a modern tyre carcass on your chasiss.

And how about some white wall?

White Wall Morgan 4/4 Tyres