I will tell you about the different styles of conversions here. There are mainly three different types, the Cali Conversion, The Down the Middle, and the Porsche style shroud. For the most part these are what is used the most.
To start with what I am doing, the "Cali Conversion" uses parts that you can buy or get from your local parts place. The main idea behind this conversion is to make it as close to the original T1 design as possible. With this style you will combine some T1 and T4 parts and engine tin to get the desire results. If you don't know what to look for, it would be easy to mistake this conversion for a T1 motor. This is also the cheapest to do, unless you are given the parts for one of the other styles. It is not that hard to do if you have some experience welding and fabricating some stuff.
The next style, the Down the Middle, was first available from Oregon Performance Parts. They designed and made these for a time until the main person behind them died. The molds and designs were then bought by Jake Raby of Raby's Aircooled Technology. That site is for his engine building company, there is another one for his shrouds. He now makes the shrouds for the T4 and from what I have heard there is one for the T1 if you are going that way. From his tests, it can be proven that this is the best system on the market. It may not look as cool as the Porsche shroud but it will work much better. I'm not sure how much this system costs (I'd have to look it up), but if you want the best, get this. From talking to him at a show and on the forums, this could be put together in a day by a semi-experienced mechanic. You would need the long block, intake, and exhaust though to make it work.
The third style of shroud is the Porsche style shroud. This is the coolest looking, but from what I have heard and seen, this type has no form of air direction in the shroud. This will not help with proper cooling. And since there are no flaps or anything you can't run a thermostat, so the warming up time will take longer, and in some cases the motor can be over cooled leading to not as good of performance.
There are probably other style shrouds out there that I haven't heard of yet. But unless they come up with something better than these, you probably won't hear of them either.