Sprint set up for SIP Road exhaust

Hi
I purchased your SIP- Road exhaust for my 1969 Sprint, I am happy with the construction and fitment but I have a few set up issues.

Some background information, I purchased it about two years ago via Ebay from Vietnam. I had heard all the Vietnamese horror stories but took the risk and have been happy with it. Had it checked over by my local mechanic when it arrived and he said it was ok and in fact one of the better Vietnamese sprints he had seen. (Although he was not, and is still not, a fan of Vietnamese Vespas).

It’s been running well but only done low kilometres (1500)with the only issue being it needed a 12 volt regulator installed to stop it blowing tail light globes which was done a few months ago.

When I installed the SIP Road pipe and took it for a ride around town (no faster than 60 kph) I noticed it missed slightly occasionally. I noted your recommendation to check the main jet and timing and arranged to take it to my mechanic.  When I rode it to the mechanic the missing got progressively worse and started backfiring repeatedly. I change the exhaust back to the stock exhaust and noticed no real difference. My mechanic installed the new pipe, adjusted the timing and carburettor settings but did not up the main jet as he didn’t think it was necessary just for a new pipe and replaced plug and points. Although this helped it still misses and backfires.
My engine number is VLB1M-0102043 which I assume is a two port engine which has not otherwise been modified. Timing was advanced to 19 degrees. Carburettor is a Spaco SI 20/17 with a 98 main jet.

Can you please advise,
1/ Whether this carburettor is sufficient or should I upgrade? What would you recommend?
2/ What main jet would you recommend for my carburettor? Any other changes to the carburettor needed?
3/ Is there another cause I should look at?
4/ Is it possible that the exhaust manifold could be leaking? Should I use a manifold sealant?

Thank you for your time and help with this

Regards
David