I also wanted to fit a 1.7 8v engine to my Sprint and I got one quote of $1000 and he couldn't assure it was in good nick so I said no but it would be different for you because you could go and have a look (you see I was going to get him to send it to me in Tasmania). I was recently in Melbourne myself and whilst on holiday I went to a few Alfa shops and parts places so here are the ones I have retained business cards from.
Here are a few Alfa people in Melbourne:
Alfa Parts New and Used Parts and Service
46 Alex Ave,
melways ref.: 78 B7
The name of the guy is Garry Oates and he does Alfa, Lancia, Fiat and would you believe it Lada. But he is good he is the guy who I go the $1000 quote from.
Mille Miglia Motors (Alfa Authorised dealer)
329-331 Edwardes Street,
melways ref.: 18 E5
This guy has alot of new stuff and if you want to pay official alfa prices go there but he charge something like $260 per water pump and I got a QH pump for $100 (and that was in Tasmania). He seems quite helpful though. He does seem to be a little biased towards official Alfa stuff and keeps harping on that the cheaper stuff is crap. But he does do the large badges for $40 (they are the non-official ones). He is open till 6pm most days though and I think he is open on a Sunday. He is definately open on Saturday. He also has a great number of useless accessories.
AlfaWreck New and Used Alfa Parts Specialists
20-22 Blaxland Avenue,
Mobile (after hours): 0413998091
Melways ref.: 8 D10
Ask for Michael becuase he is the Alfa guy. By the way it might be advised to get your new parts here as he charged me $25 for the rubber boot that goes around the gearlever (not the top one but the bottom one) and Mille Miglia Wanted $70 even though he couldn't get them anymore. So I think that they might be cheaper and better. I am not sure what there used parts area is like but they might have what you want.
There was another one in Thomastown that was really quite close to Blaxland Avenue. They are in Tuscan Court (Melways ref.: 8 G10). And they are up the right hand end you will see them because there will be an Alfa or Lancia or Fiat parked out side. They have a wreck yard just down the road a little further aswell. Iam sorry but I don't have a business card for them and as such don't know their phone number or trading name. But if you go to AlfaWreck you might aswell just go up the road that little bit further to find them as they might be able to help you.
I think that AlfaWreck and AlfaParts are your best bet.
You should also think about what gearbox you want, the 1.7 or the 1.5 one. The 1.7 gives a higher top speed but the 1.5 gives better acceleration.
I think that you should also post your question here http://www.alfa-pages.co.uk/?go_to=disc
Hope this helps,