I guess you already know this, but the SL engine underwent some big changes between the K0 and K1 models - it lost the electric starter, got smaller carbs and a different cam profile. The K1 ended up about 6hp down on the K0, but it made up for the loss with the weight saving from frame changes and dropping things like the starter.
As some others have mentioned, the K1's had slide carbs whereas the K0's had CV's (the slides gave better/more reliable response off-road). IIRC the K1's generally had 28mm throats, whereas the K0's were something like 32mm, and from the microfiches, it looks like the heads were different to accommodate the different inlets. Assuming they were, you might need to do some fabrication work to get the CV's to fit - or find yourself a decent K0 head on eBay. Don't forget that if you get the earlier carbs, you'll probably need new cables (the throttle linkages are different) and you may also need to sod about with the air filter tubes/connectors. Once you've got the bigger carbs on, you can look at rejetting and stuff to see if that'll give you any extra grunt.
With the exception of the camshafts (which, incidentally, changed during the K0 run - mid '69 - and then again when the bike was restyled for the K1's - mid-'70), all other top end internals between the K0 and K1's seem to be pretty much the same (valve sizes and so on) so a straight swap shouldn't be too painful. And if you're into experimenting, I guess you could also try to find yourself a K0 cam, which IIRC should give engine performance more akin to the CB/CL's than the off-road oriented later cams.
Think that's pretty much it for the simple-ish stuff, other than electronic ignition, messing about with the engine timing and putting on some road tyres . . . BTW my K0 runs pretty well at 8-9,000rpm without sounding like I'm torturing it too much, but I wouldn't want to sit in that sort of rev range for any length of time
