There are many different editions of Runequest. I started playing Runequest back in high school. The GM used a combination of Chaosium's 2nd edition and Avalon Hill's 3rd edition. I didn't realize this till I purchased the Avalon Hill rules myself. I tried to run a Runequest game using the same combination of 2nd and 3rd edition a few years ago. It didn't work for me or my players. I didn't try and run Runequest again until years later. I purchased the Mongoose Runequest II rulebook at a convention. I liked what they had done with the system so I started up a campaign. I have a page for it over on Epic Words.
Mongoose Runequest II (RQ2) uses slightly different stats from original Runequest. Here are my notes on how I convert between the systems. RQ2 and Runequest 6 use similar stats so these conversion notes should work for RQ 6 as well.
I don't get detailed with my conversions. I convert the combat statistics and magic. Anything else can be done on the fly. If an NPC has a skill that doesn't exist in the new system, simply give them an equivalent skill.
Converting Combat Stats
The base stats of STR, CON, SIZ, INT, POW, DEX, and CHA all remain the same. Use these stats to compute the creatures Combat Actions, Damage Modifier and Strike Rank according to the tables in the RQ2 rulebook. Armor, hit points, and locations hit points all remain the same.
Original Runequest had a separate skill for each weapon. RQ2 uses combat styles. The creatures highest combat skill can be their combat style.
The Evade skill replaces Defense. I use DEX x3 for a starting Evade and then adjust it up or down.
The Brawn, Resilience, and Persistence skills replace the function of the resistance table. Brawn is used for contests of STR. Resilience resists physical hazards like poison and disease. Persistence is for defending against magic. I use STR x 3 for Brawn, Con x 3 for Endurance and, Pow x 3 for Willpower. I adjust these up or down slightly depending on the type of NPC.
Magic is more difficult. There are many types of magic in Runequest. For common magic, I don't convert the spells directly. I give the NPC a common magic skill of Pow x 3 and use the spells as written in RQ2. There are a few spells that don't exist in RQ2. I simply ignore it or swap it for a different spell.
Divine Magic relies on the Lore and Pact Skills. For a one shot NPC, I don't worry about the Pact skill. I use INT X3 for the Lore skill. If the NPC is a priest or a runelord, I would build them from scratch as a new NPC rather than try to convert them.
I don't know of any original Runequest supplements that have a sorcerer or dragon mystic. If you need to convert one, I would build them up as a new NPC.