Would a motorcycle that was originally designed to run on Regular gas have to be modified to run on Unleaded?

depends some on the actual cycle. 2 stroke oil at 100:1 is good enough for BMW R50 and r60/2 because of the valve timing has lots of seat time, no overlap. R69S needs the stellite seats and hardened or knurled guides. Harley FLs can get by awhile on the 100:1 2 stroke oil, FLHs and sporties need the later valve seats and guide work, valves etc. 1970s Yamaha 2 strokes? no problem , just run the unleaded premium WITHOUT the ethanol. Honda 350? additive and don't run 6000 rpm all the time. Some of the European cycles of 1960s, early 1970s were meant to run on a very low lead or unleaded gasoline- note the BMW R50/2. BSA had some low compression models with fairly hard seats, moderate Stellite or nickled Iron and Enfield also had the low lead,low octane versions- these are sometimes called 'colonial export', 'African Commercial' , Mexican Market models.

The smart thing todo is use the additive. Messing with valves when you don't know what you are doing will cause major damage.

The valves would need a different angle on the seat and valve and that alone would mean a complete tear down of the engine.

The additive is so much better

I have a 76 and run unleaded, no problems at all. It doesn't bother a Honda, no modifications, no additives

unleaded conversion is the best way to go,stellite tipped valves and guides. simpler,otherwise you have to have the additive with you all time who needs that? Do the conversion and forget about it

If you get the hardened valve guides the additive is not needed.

Rings need replacing as they where designed to lube with leaded gas.

The carb will need to be rebuilt or tuned up a least.

Valves if you want to.