My advice is the same as Powell's - bite the bullet and buy new sections, especially if this will be inspected.  Size 5 starters are big and generate a fair amount of heat.  I wouldn't even consider trying to retro-fit a 45 year old MCC section for new Size 5 starters.  Who knows the condition and rating of the vertical bus, horizontal bus, ground bus, etc?

I'm guessing it will be cheaper (eventually) just to buy new sections.  Maybe get rid of the whole thing while you're at it.

Let me guess - a sawmill maybe?  

It is for a natural gas fractionation plant, we have already tried to talk them into replacing the ting but the client won't budge.

Try to convince them it will be cheaper - that usually works.  Otherwise, you're going to have to get a UL508A shop to come out and build up the starters in place, I guess.  It will be ugly.  You may need UL field inspection depending on what state it's in.  

Also, re-using the existing sections will likely require a longer outage than buying new.  

You could throw in something about safety as well.  

I'd be willing to bet 1/2 of a paycheck that buying 2 custom built size 5 combination starter buckets for an MCC made by someone who stopped making MCCs 20 years ago, AND getting them UL listed, is going to cost 5 times the price of 2 new MCC sections with new Size 5 starters in them from any make.

Besides, most likely they were not stab-in were they? The usual limit on that is Size 4. So if they were bolt-in, why not just replace the bad components with new ones anyway? In fact, a good argument could be made to replace those behemoths with solid state soft starters.