Ethereum Classic hashrate is 1.0 Th/s (Scypt). This is about the equivalent of 40,000 high-end GPUs. To get the amount of hashrate to perform a 51% attack (for the purpose of double spending) you'ld need to control about $10M worth of GPU hardware. This exists in the supply chain (new GPUs), or could be redirected from other altcoins, where as many as 20X this number of CPUs are already in use for other altcoins. For instance, Ethereum has about $80M worth of the same GPUs. So if just 1/8th of Ethereum miners were to decide to temporarily re-assign their hardware, contributing the hashes to a 51% attack on Ethereum-Classic, that attack would be successful.