Heya Myke! Nice score here! I am far from an expert, so don't take my word for gospell, but I've been looking at pictures all morning. And will do some speculating.
Is there a letter at the beginning of the serial? Sometimes an M? From what I have read, there is a letter, then the first digit is usually the year. So I would guess this is an 81.
The seat may have very well come from a later 80's Californian. They seem to run that.
The spyder may have come on a this goose, but I don't think the chainring did. Is it stamped? The crank arms should say Mongoose if they came on it I'm gathering. But they look to be Cro-mo tange, so a groovy swap.
The brakes could be correct to the bike.
The pedals are later add-ons. (What are they stamped as?)
The stem and grips are later too. But the bars almost look like they could take a double clamp stem. Which would be the stem on the bike originally.
The seat post clamp looks later too. (Is it stamped MX?) Maybe it came from the same place as the seat? Possibly the bars? I wonder if the builder had a donor bike with the red bits? (Any stamping on the seat post?)
I am not sure about the fork other than it didn't come on the bike originally. The early gooses all seem to be bottom loading forks. Is it stamped?
Hopefully someone will chime in and correct all my mistakes! (Again I am not an expert on gooses, just curious and have google in front of me!)
It is a really sweet bike! Someone rode it and upgraded/replaced as they went. I call that mojo. Congrats!