Don't really know anything much about mini choppers and can only suggest that you phone a few bike dealerships and ask them. If they don't do the service they may know someone in Sydney who does.
http://www.yellowpages.com.au/search/listingsSearch.do?region=australia&headingCode=12769&sortByDetail=true&sortByAlphabetical=false&bookId=1&businessType=Motor+Cycles--Parts+%26+Accessories--W%27salers+%26+Mfrs&sortByClosestMatch=false&areaId=1054&rankWithTolls=true&sortBy=mostInfo&sortByDistance=false&stateId=1&safeLocationClue=sydney¤tLetter=&pageNumber=1&locationClue=sydney&locationText=Greater+Sydney+NSW&serviceArea=true
Here is a link to yellow pages and motorbike shops
Also try posting this in the motorcyle section under cars and transport as this section is for racing so you may not get many answers.
http://www.yellowpages.com.au/search/listingsSearch.do?region=australia&headingCode=12769&sortByDetail=true&sortByAlphabetical=false&bookId=1&businessType=Motor+Cycles--Parts+%26+Accessories--W%27salers+%26+Mfrs&sortByClosestMatch=false&areaId=1054&rankWithTolls=true&sortBy=mostInfo&sortByDistance=false&stateId=1&safeLocationClue=sydney¤tLetter=&pageNumber=1&locationClue=sydney&locationText=Greater+Sydney+NSW&serviceArea=true
Here is a link to yellow pages and motorbike shops
Also try posting this in the motorcyle section under cars and transport as this section is for racing so you may not get many answers.