The project fell into Mr Crossan's hands when the original model engineer working on the engine died in 2012.
Mr Crossan - who has 50 years' experience in model engineering - received the 1/4 scale World War 1 vintage Bentley BR2 rotary aircraft engine ''75% complete'' and ''all in pieces and boxes''.
Additional parts still needed to be machined and the engine assembled.
''I never counted the hours,'' he said of the work involved in completing the engine.
''There were too many, but it was off and on for two years.
''I had to make the rocker-heads and pushrods and the cam gear.
''There was a lot of putting together and then re-putting together until I got it right.''
He patiently persisted with the project until it was running to his satisfaction earlier this week.
''When it was running it sounded smooth and beautiful,'' he said.
''I was amazed. Putting this together, there's so many parts it's a wonder it went at all.
''I had no idea it was going to perform.''
Machining the rockers had made him question if he had bitten off too much.
''It was just about as much as I could do,'' he said, of the difficulty of machining the rockers - which are measured in millimetres - from high-quality steel.
Many of the parts required specialist tools, which also needed to be created, he said.
But the result was gratifying.
''I'm quite pleased with it - it looks quite good,'' he said.
''This is all the stuff you see - but there's stuff in there that you don't see and it's quite amazing.''
The difficulty of the project had made him admire the engineer who began it.
''I slowly came to admire the guy for just taking on the job.''
The model will be on display during the Otago Model Engineering Society's festival week display at the end of the month, before being handed over to the Air Force Museum of New Zealand.
