Re: Engine swap?
I get what you're trying to do. Not the "usual" motor swap but certainly an understandable one.
Here's an idea, that I dont think anyone has mentioned before (and my apologies if they have) - if your frame is "tradeable", why not swap it for a 600/750 frame? You could probably get a similar condition frame + cash. Some of the guys out there looking to do the usual 600/750 frame / 1000 motor swap might love to have that frame, and be willing to pay some money to get yours & avoid the usual frame/1K engine swap problems (like having to cut/weld/fabricate motor mounts or using a mounting kit)
By "tradeable" on your frame, I am referring to whether you have some paperwork with your frame. Salvage title, clean title, bill of sale.... etc....
If that paperwork was NOT available - for whatever reason (and I'm not going to start thinking of why not) - then yeah, you are stuck with the frame you have.
But really, if your frame has some paperwork & is in decent condition you could swap it for a 600/750 frame, and get some cash to help pay for the new motor.
When its all said and done though, if you are determined to keep the frame, I'm guessing... that if you are careful with what you buy, and hunt around a lot, then you might be able to finish off your K5 with a 1K motor for probably 500, maybe 1000 bucks less than if you went the 600/750 route, and i GUARANTEE that the resulting bike will be worth more than that difference if you ever have to sell it!