Ironically that engine will more than likely be fixable, engine rebuild is an engine rebuild. Stripped cleaned new clutch and rebore, bit of vapour blasting and a crank rebuild and its as good as any other. Frame on the face of it seems ok. Sw arm is scrap but the shock will most likely be good. Hubs too will likely be ok as well. No I don't think its a write off if your enthusiastic and the bike has sentimental attachment, like mine I have had it getting on 30 years. But you can buy a working dt with mot for under 500 quid if you just want it running, can you rescue that with that amount of cash, ??