Ossa, that is one pretty bike, don't know much about Ossas but whoever designed that had a good eye, the flow thru tank, seat base, rear guard and side plates, the way the aircleaner fits into the cowling, the centreport engine.
Bet you spend some time on Ebay Spain, sometimes I go there when I am dreamin' of Metrallas, sort of like mechanical se... better not say it.
Post some photo's as its coming along, will be good to see no matter how long it takes.