Swapping MAS19 BS7 for a SH1/18

Hi,
My stood-for-decades Series 2 arrived with a clean but very tatty looking and non-standard SH1/18. Cold starting is a pain as although it fires on the second or third kick, it dies almost immediately. Once it's warm it runs pretty well and starts first kick. The choke appears to opening fully but it is still a pig from cold.
Would I save myself a lot grief by putting a new MAS19-BS7 carb on it? If so, do the MAS19-BS7 and the SH1/18 fit the same inlet manifold, even though they are different venturi sizes?
Cheers!
My stood-for-decades Series 2 arrived with a clean but very tatty looking and non-standard SH1/18. Cold starting is a pain as although it fires on the second or third kick, it dies almost immediately. Once it's warm it runs pretty well and starts first kick. The choke appears to opening fully but it is still a pig from cold.
Would I save myself a lot grief by putting a new MAS19-BS7 carb on it? If so, do the MAS19-BS7 and the SH1/18 fit the same inlet manifold, even though they are different venturi sizes?
Cheers!