Being realistic and discount any nostalgic value, you have 2 choices.
1- if it is basically OK and you can find the needed other parts in a scrap bin someplace (not very likely given that it's 40 years old) you can get it working.
2- otherwise, you can keep it as a piece of vintage hardware and use it as a decoration or paperweight.
In theory a restorer might be able to make the missing parts, or repair the head if it doesn't work but it would be extremely costly.
If it has nostalgic value put it in a safe place as a keepsake, and buy a modern unit for $20.00 or less for the bike. Or if it doesn't have any nostalgic value, toss it and move on.
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