I had a look in the workshop at my spares, and I can build you up a new UDP 4 Port (complete except for head shims).
I have other UDP spares left over, so can offer you the following for NZ$200 plus postage:

New UDP 4 port engine as above, plus

New spare UDP crank
3 x new spare UDP con rods (slightly different to the 21-0600 I believe)
1 x new spare cooling head

Let me know if you want to proceed and I will build it up for you.
Email me at [email protected] as opposed to a PM (I don't log in on here all that regularly these days).