the info on replacing it's in the wiki:
I think by your carb # you have an A cylinder so you can't simply flip the intake forward. you'll need to make a spacer (i've used just extra intake gaskets) and shims on the clamp mount (you can use cut up sody cans) and it'll fit right on, tho maybe at a silly angle
The Dellorto SHA is a very close fit, and with some simple modification to the intake it will not only fit on the engine, it will also still fit under the side covers. There are a couple different ways of doing this,but before you start check the fit of the carburetor as due to slight model differences and casting variations it may already fit. The first way requires no skill or really anything other than wrenches. You start by removing the two intake bolts and washers. Next you remove the intake and reinstall it with the two washers, both on the rearmost bolt between the intake and the intake port. This should angle your intake perfectly, allowing the carburetor to slip on easily, clearing the top of the engine. The last step is resolving the problem with this method; that's sealing the big gap left over. This could be filled with plumbers putty or a similar compound so long as it is both heat and gasoline resistant. Plumber's putty may be neither so read your labels well!
The second method will take a bit more time and is more desirable as the likelihood of sealant breaking loose and being sucked down the intake is much less. This involves making a spacer, for this I chose 1/4" Masonite (basically the cardboard stuff that goes on the back of your entertainment center). It's derived from paper pulp so it should compress and behave the same as a paper gasket. You start by tracing around the base of your intake and then cutting it out with a fine toothed hand saw (hack saw works, coping saw works even better). Once it is cut out and trimmed up nicely, get a stamp pad and push your CLEAN intake into it and then "stamp" the whole pattern onto the masonite. Start by cutting out the center whole by drilling in the middle of it then grinding the rest of the material off with a Dremel. Next use a C-clamp to fasten the spacer to the intake and a piece of scrap wood underneath making sure your holes are lined up. You will now use the bolt holes in the intake as drill guides so as to attain the proper angle, choose a close fitting drill bit and drill through the spacer and into the scrap wood. Once you've cleaned up all the little "fuzzies" and bits of peeling paper, your intake spacer will be finished! Now you need only mount up your intake with your new 1/4" thick gasket. This moves the carburetor up and will clear the engine case and should clear the frame. In some cases the spacer may require some tapering to achieve the desired angle. This can be done by sanding or any other appropriate grinding method. It is also worth noting that if you are using a 15mm or 16mm carb, you will need to further modify the intake. Simply grind down the diameter of the intake where the carb attaches until the plastic sleeve that came with the carb slides on with no resistance. At that point use a sealer, or glue of your preference to fix the sleeve to the intake creating a perfect sealing mount for your new carb. Having done this on my G3 I found that it still took some fiddling to get it to clear the frame, but it does. In the case of other frames the fit may require a thicker spacer to be made or may have no trouble at all. The only other modification required at this point is to drill a small hole in your side cover to run your throttle cable.
For the C and D intakes, the mount is flat so they won't fit on an A jug. With the C/D the intake has a 14.5mm ID, and can easily be flipped forward to fit an SHA or similar carb, but may require cutting the top-right most cylinder fin.