First things first!  Please tell the MMD what parts you need.  If they
aren't too expensive you can afford to buy the parts the second time,
to get them _now_ so you won't loose the restoration job.

There could be the slim possibility that the seller will send you the
parts if you threaten with legal actions, _if_ he has those parts at
all, but the easiest solution would be to take the legal trouble
afterwards, and try to help your customer now.

Since I'm Norwegian I cannot say anything about US legal actions, but
I am sure you should claim that "the deal is off" and have your money
back in some way.  (I understand you can "sue anyone for everything"
in the USA [ :-) ] so, provided with enough resources, it is probably
possible to make a legal claim.)

I guess a universal rule would be to have concrete written
documentation about the trade present.  (If you have stated written
time limits for those critical parts, or have a letter from him telling
you about concrete dates, your case should be stronger.)

And -- depending on what you have already have used as excuses in front
of the customer, and how rare those parts are, and how (technical)
understanding your customer is -- I cannot see that it should damage
you too much to be honest and say that your supplier has cheated you,
and that this has delayed the delivery of those parts, and you work
hard to solve the problem.

This might now be easier to tell if you are lucky and someone in
the MMD community has the parts, and you can finish the restoration
soon.

Regards Thomas Henden