They can nationalize oil production. I believe SNP has pledged to repudiate its share of the UK's debt. Assuming they manage their economy rationally (like Norway), they should be fine. The rhetoric coming from the SNP suggests Venezuelan policies. If so, they're in trouble.
Repudiate on its debts? They’ll get loan-shark rates on public borrowing if they do that.
In any case, they’ll still be doing about 70% of their external trade with rUK and they’ll be outside the EU, which means we can slap tariffs on all Scottish goods coming in to rUK until the debt is settled.