Blue Water Yachts still has a few of the swing keels left:
They are about 80 lbs and the same swing keel was used on about 11 different models from 1968 through 1987 - Macgregor 21, Macgregor 22, Macgregor 25, Venture 17, Venture 21, Venture 22/224, Venture 23, Venture 24/224, Venture 25. $695 but still available in limited supply today. They also have all the pivot, lock bolt, cable tubes, etc for that boat along with a number of other parts specific to the Macgregors.
Looking more closely at picture of the Swing Keel sitting in the cradle it appears the damage goes all the way from front to back so you may even have to replace some of the steel inside and do some welding. It may be time to bite the bullet and buy a new one while its still available. The poly vinyl filler is almost $150 a gallon depending on where you can buy it and your going to need a lot of it to deal with that much damage so really price out what it will cost you to repair it yourself compared to buying a fresh new clean one. You may find that with filler, resin, fiberglass mat, steel, Ospho, fairing compound, etc you may end up halfway to the cost of replacing it.
Get into it quickly before the last few replacements are sold and that option is no longer available.