A Belgian in Spain since 1945? what was he a collaborator/black marketeer/ war criminal on the run?
Sounds dodgy to me.
I have had a lot of dealings with bikes in Spain, and indeed the peak of my dealings was in 1977-19 just after the fall of Franco.
You are up against several problems of credibility here;
After the civil war ended Franco put a block on all imports and of motor vehicles and spare parts especially. So by 1972 a hell of a lot of bikes were very badly butchered indeed just to keep them on the road.
Some were smuggled in, a lot of older stuff had been "modernised" locally, but also there were several home grown makes of small bikes, Bultaco, Montesa, Ossa and four stroke Sanglas etc
I would suggest therefore you have a choice of pre war OHV Norton singles, the Spanish forces used these, also believe it or not OHC velos.
But, more believably I think a pre war or ex Wehrmacht BMW would be much more believable, lots left there, or a home grown Sanglas 500cc single as used by Spanish police.
Older stuff except the BMW would be magneto ignition, others would be battery and likely flat.
Any large Belgian bike would have had to be pre war and therefore worn out. Of course there is the choice of some several tasty french bikes, Terrot, Motobecane, I think the credibility of a Vincent in Spain in the pre 72 era unlikely.
I exported a lot of bikes to Spain at that time, never saw or heard of a Vincent there, just about everything else though as before Franco Spain was a big Market for Brit exports.
A 1000cc Sarolea? thats a new one on me, after 1945 they made a 500 Sarolea Atlantic twin, I think it was, only a handful though and shortly after they were more or less defunct.