Thats a shame, if were on this side of the pond you would find me on your doorstep! i would take a lot of pictures and stick it on eBay with a starting price of $1000, it's got to be worth that even in the state its in. Restored it would be worth $4000 to $5000 everyday
The bike is a replica of the MZ works ISDT bikes which were unbeatable for much of the 60s and early 70s. I would pop round your mates again see if there are any other bits lying in his scrap pile ( seat and exhaust being the most obvious, and even if badly damaged they would be useful for patterns) . Here's a picture of what you could have had
Well this is actually a 1970 bike but it's just about identical. And the front brake plate is like yours so thats original. The twin plug head was so that if one fouled, you could switch to the second one without losing any time, the switch is under the tank at the back.
ES250 Doppelport, ES250, ES250/1, ES250/2,ETS 250, ES150, ETS150, BK350, IWL Pitty, SR56 Wiesel, SR59 Berlin, Troll............ and thats just the German two strokes!
http://thecomeconcollection.blogspot.com/