The Spaniard, whose car flew over the Sauber of Monegasque rookie Charles Leclerc at Spa after being shunted by Renault's Nico Hulkenberg, said the Monza chassis was one raced earlier in the year.
He hoped the Renault power unit could be salvaged but would not know for sure until Friday practice at Monza -- a high-speed track that under-performing McLaren had expected to be difficult for them anyway.
"We want to put it on tomorrow and confirm that everything is OK. There were some concerns on Sunday, then Monday and Tuesday they checked everything properly and it seems OK," he told reporters on Thursday.
"So hopefully we can run still with that engine," said the double world champion.
Of more concern was a shortage of the latest specification parts, with Italy following immediately on from Belgium.
"The full car -- not only the chassis -- the floor, the front wings, the things that we are limited on parts they are gone. It (the crash) was quite expensive," he said.
"Probably we will run out of spare parts. We have just what we have on the car. All the rest will probably be a different specification."
"This is quite tough, because the damage on the cars was quite extreme, especially on my car. And then you have four days only to build a completely new car for Monza."
Alonso's Belgian team mate Stoffel Vandoorne replaced engine parts, with consequent penalties, at Spa while the Spaniard did not in the hope of a points finish.
Instead, the crash ended his race at the first corner and he will still face engine changes and grid penalties down the road.
"It's a double zero; one in Spa because of another guy, and another one that you will take the penalties. So quite a sad day," said the 37-year-old ahead of what will be his last Italian Grand Prix.
Alonso, the only driver on the current grid to have won at Monza for Ferrari, has announced his departure at the end of the season.
He said the pain of the crash, which had left him with a sore wrist and back, had subsided.
"The only painful day was Tuesday," he added. "On Tuesday when I woke up I felt a little bit sore in the back.
"I'm still feeling some kind of heat (in the wrist), because I had the hands on the steering wheel when I touched Leclerc. But every day it's less and less, and it should be OK for tomorrow."
(Reporting by Alan Baldwin, editing by Toby Davis)
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