Hi VC Victa, Geez,$600NZ to fix the prob....you could probably buy a brand new machine for that price.
However, let's look at the prob. You say that it will start ok with "Start Ya Bastard" These ether based starting sprays are definately
not recommended for small engines and damage can result from continued use.

The workshop has indicated that the piston is badly scored; well, if that is the case, what does the cylinder bore look like....it could be badly scored as well and will require a re-bore/re-sleeve plus an oversize piston.
As for the rings, are they still intact or broken (which could have caused futher damage to the bore.
And you would have to look at the condition of the head, seals, bearings, gaskets etc...You can see the $$$ piling up plus labour costs.
Plus the fact that Tecumseh engines are no longer manufactured (they went into liquidation)...are spare parts going to be available in the future?
In my opinion, you may be better off looking around for a good second hand engine that would suit, and can be had for the
right price.....especially if you are going to sell the machine.