I'm a huge fan of the Tutto Lambretta stock exhausts and have them fitted to 2 different engines. IMHO they're the next best thing to a NOS exhaust, and many people would be hard pressed to tell the difference. Casa make some decent parts, but standard exhausts are not one of them. The last Casa exhaust I purchased was so dire I personally returned it to Vittorio for a refund. I'll try to dig up some photos I took of a Casa side-by-side with a Tutto, the difference in quality is shocking, particularly given that the retail price between the two is comparable.
I've not yet seen the Scootopia exhausts yet, but I've had a 'proper anorak' friend visiting from London this week, who occasionally road tests pre-production parts for Scootopia, and he said they were decent. Speaking of Scootopia, back when they were a Casa main agent ever notice they never stocked the Casa exhausts, but the Tutto ones? To me this speaks volumes.
If you purchase parts from Tutto, I find its best to order direct via phone (speak to Cinza). Alternatively, you can purchase online from STF.
Regarding the 'mushroom baffle' this changed throughout production. For example, the inlet holes in 'mushroom' were bigger on the TV/2 exhaust than the LI/2 exhaust and obviously it was abandoned for late SX and dl/GP.
Personally, if it was my build, I'd purchase a Tutto S3 exhaust, dry build, and then follow recommendations in the '60s Performance and Tuning Conversion Manual and drill the out the mushroom with a 13mm bit, before cleaning and sending off to be ceramic coated with Cerakote. In the past I've also had success removing tailpipes and re-welding them on, but given the Scootopia S3 'clover' ones are built to late SX/dl/GP spec there is no need to do anymore.