Why is a very good question... and without sounding rude, if you have too ask, you need to go speak to a garage.
You don't simply 'replace' a piston. You're talking full blown engine dismatling, and if you're doing it properly, that includes it coming out to be stripped and re-bored. Pistons don't generally fail individually, to be honest they don't tend to fail at all. Bearings maybe, big or small end, but 9 times out of 10 they will show the effect and not be the cause.
Bit of a ramble there... but you gotta start with why you think you need a new piston.
Pistons do fail individually - usually as a result of lean mixture in that chamber, most likely related to a failed injector.
Procedure is head off, sump off. Undo the big end bolts and push piston up. Measure cylinder for wear, if it doesn't need a rebore or a hone - which would be related to wear, not damaged piston unless you are unlucky and a part of the piston has broken off and is bouncing up and down the chamber scoring the cylinder walls.
Get new piston, new rings - size all dependent on the wear of the cylinder. Fit the rod to the new piston, get new big end bearings, refit by sliding piston down into the cylinder and use new bolts for the bearing cap is I'm sure they are stretch bolts.
Easy, done it many times on various engines following a single piston failure. Mostly on turbo engines though, not NA.
The piston needs to be replaced as it was damaged when the tip of the spark plug broke of late last year the car was stripped down and damage was found on the cyclinder head and on the top of the piston, i have just had the head done at cylinder head engineering in portsmouth, (which have done an excellant job) but whilst the damged to the piston could probably be carefully ground back and not re-placed as the head has been done and new cambelts will be replaced etc i don't want the thought of this slightly damaged piston failing so thought if i could easyly replace it then why not, i just need to know how, there is no damage to te bottom end so just of and on again, i hope this clear things up.
How bad is the damage on the piston, can you post a picture of it? You should be able to get just a single piston and the rings which wouldnt cost much, and you should hone the bore to help the new rings bed in. Whilst the head is off obviously now is easiest - drop the sump, undo the bearing cap and push the lot up. Then just give it to an engineering place and let them worry about fitting the gudgeon pin as you will need a press.
Ive had several cars with individual piston damage as a result of overheating usually broken on the edge by the recess for valves.Alfa t sparks are prone to end up with bottom end problems after individual or partial rebuilds by owners/garages. logic suggests they shouldnt but history shows they do.i would be inclined to think carefully about a rebuild.im not 100%sure but i believe the piston will come complete with the con rod as they are manufactured as one unit.
In fact I don't think you can get a single piston from Alfa as they are made in batches, and I believe the set has similar weights within a certain tolerance. The likes of AE/Federal Mogul should be able to sell a single piston - find a good engine machine shop and they'll be able to find out, order it, and move the gudgen pin and rod over.