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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

is it a TV or GP crank bud?

Any doubts, reset to 17 degrees and it should run cooler.

Over-boring will have bumped up compression slightly, which normally requires ignition advance to be reduced. Modern fuels burn hotter, which requires ignition advance to be reduced.

Standard T5s, in the days of better non-E fuel, ran something like 15 degrees btdc so please don't think of 17 being in any way extreme!

FYI - A pal took his reliable Euro Lambretta iron 175 to a dealer to have his seals replaced and the dealer mentioned that he had "corrected" the 17 static timing to 19 degrees. The result of this 'correction' was that he seized it solid on the Newbury bypass, nearly missing the Euro! Piston replaced and timing put back to 17 dbtdc and it didn't miss another beat. On the return leg he covered 430 miles in a single day.