Series 3 Standard Engine ignition timing?

[Littlewheels]

I've been running my newly rebuilt engine with the ignition timing set at 19deg using E5 petrol. It runs ok but on longer rides it gets really hot and I'm worried it may seize
It's a TV175 with standard head, barrel, piston, and standard 20mm carb. On the main jet i've gone up 1 size to 107 as the barrel has been bored out to 181cc. Oh and it's running on a standard 6 pole points ignition system.
Any help greatly appreciated