How much offset is it?
i had the same problem thought about offsetting the motor but the ones ive seen done didnt look great cant imagine they handle to well either ,i went for the jackshaft option like dinnertimedave suggested , looks cool as long as you have the space ?? theres some pics of mine on my page have a look matey .
I'm not really understanding your problem because I've never built a Sportster. I had to use a sportster sprocket on my shovel because they are flat and four speed sprockets are dished. The difference was all it took to center the wheel in the frame. Here's a stupid question, what if you switched to a 180mm tire? Would that give you clearance? Like my friend Steve says "there are no problems, only solutions".
I've just had a look at Shane's pictures---very nice!! Beautiful work mate, thats got to be the easiest (and nicest) way to go Jocke, like the inboard brake keeping the arse end clean too.
Built a sporty a couple of years ago. Road 6 customs weld on, 83 chassis with an early 90's motor. I was able to use an offset primary sprocket. late model 5 speed motors have available to.750 offset. I was able to use a .500 offset to run a 170 tire. Early 4 speeds have a lot less choices but a good machine shop offset my brothers early sprocket. 450 by cutting out and bulding a sleeve. Check my page, you will see my Hemi orange chop build there.