If you go to twinport you'll loose some of that lovely low down torque, but you'll gain a bit more power higher up.
If it was me, I'd say keep it singleport and find the right stock solex to suit it instead of the 1200 one.
However, if you do go twinport, remember that to do it right is more than just buying a pair of TP heads. You'll also need twinport cylinder tins, inlet manifold, doghouse fan housing, wider fan, oil cooler and bracket, flap linkage, two piece doghouse tin and rubber seals for it, hoover bit, and new rear tin (over the bellhousing). You will also need to fit 4 shorter head studs (upper centre ones) The cash and hassle adds up.
And if you then slap a progressive on top of it you'll have a new set of hassles trying to get it to work well with the aircooled, as they were designed for ford cortinas and the like, they are not well suited to aircooleds and won't give you much power increase for the hassle and cash spent. They can be particularly troublesome if you want to run them through winter on a daily driver as you can't fit the proper VW air filter and therefore they are prone to carb icing unless you DIY bodge up some hot air ducting to feed the air filter. A few people have managed to make them work well and get a few more HP but they'll never give you the sort of power increases that twin carbs will because they're still crippled by the very restrictive "handlebar" manifold setup.
If you want carb upgrades, go for proper twin carbs instead, they will give you more bang for your buck, potentially better running and less icing hassles in winter. Aftermarket carbs that are centrally mounted are a halfway house, not good for one thing or t'other. If you want a single carb best stay with the factory setup and it's stock air filter. If you want twins and revvy power then convert to twinport first and then fit separate carbs, either ICT's or DRLA's / IDF's depending on the thickness of your wallet.
Whichever way you want to jump, cost it out carefully before you set your plan and bear in mind what has been said above about upgrading the top end on an old bottom end, if your case and crank have had a hard life they won't last long when you want to rev it higher and push more power out.
Dave.