Hi Gui, pleased to here it's running respectably.
If it was perfectly OK before the blown gasket - and you're absolutely certain that the electrics & sparks remained unchanged; -
then it can only be carburration...
As I recall; - the Solex Bi-start has no less than SEVEN fuel or air jets - all of which need to be completely clear for faultless running.
I suggest you strip the carb & give it a really thorough cleaning - air blast each jet (don't poke with wires!).
Xylene is one of the few solvents which will dissolve petrol deposits - maybes soak all parts overnight before air blast & reassembly.
Petrol deposits??? - yes such do occur - particularly if laying dormant over winter - and far more prevalent with the modern high ethanol content crap which comes out of today's pumps - it leeches various components out of the Mazac / low grade aluminiun trash that Solex used for post-war manufacture.
I replaced my Solex with an Amal 626 - the "sticktion" as is rumoured to be caused by high induction pressure is simply avoided by overstreaching the return spring. (Note: I do not concur with Draganfly wrt jet sizing of this carb on a Mk1 Sq.).
Final suggestion; - try the occassional use of Tetraboost http://tetraboost.com/product-info/
every third or fourth tank refill...
PS. Stuff the Covid "crisis" - gonna 'phone Hamlin's today to chivvy them wrt progress on my short block refurb...