Modern Vespa

Howzit guys, hope someone can assist me. I'm struggling to get an old replacement engine to start easily and is running hot which would point to it running lean / suffer from an air leak as it has on two occasions soft seized while riding at 50% of throttle - at a speed of roughly 50 to 60 kph.

The question I want to ask is this. I've noticed that it splutters fuel back up the carb when opening the throttle. I've measured the space between the rotary valve and casing and this is in excess of the recommended 0.05mm which could point to the excessive air intake and erratic idle and poor starting because of the inability to suck fuel into the chamber - yes? Is it possible to have this fixed by having the casing welded? I've even heard that installing a leaf spring valve typically found on LML's could assist.

Can anyone assist with a perspective here. Footnotes as follows: jetting is 160 BE3 118, idle is 45/120 instead of the standard 55/160 to run it a little richer. I'm mixing oil at 4% currently as I live at the coast and running a B7ES plug which looks light chocolate brown on a chop.

Yes a rotary pad can be welded but the harder (more expensive) part is having it machined. But you don't know until you ask.

There is a reed valve available instead of repairing the pad. I think it's made by DRT? From memory it does say that there is fairly extensive modification required to fit it, however once it's done you should have more power and it will be almost invisible.

Why 4% ? It is my understanding that more oil displaces gas and therefor creates a lean mix.

As well as a weaker mixture. Oil is hard to compress. Extra 2 stroke oil makes extra heat which will increase the chance of detonation.
Our engines could possibly run ok on 1%, so 2% already has the safety factor added.
The rotary pad has to be really very bad to cause any issue. Run the fuel down and refill with 2% and see what you're working with.