Here is an old study back in 1977, describing various drugs that were tested to reduce ketamine induced delirium and unpleasant dreams.
Lorazepam was found to be effective and was recommended as the overall best choice. However you will note several other drugs that might be more commonly available for administration prior to ketamine. Droperidol for example reduced severe delirium in majority of cases, albeit unpleasant dreams were still reported by 30%. Promethazine is widely available and reduced severe agitation dramatically but still 20% reported unpleasant dreams, albeit none reported refusal to have ketamine anaesthesia again.