First, I think that they are two expensive for what they do, mainly do to the WS 3 BS3. And second, I really don't know if I am a good enough scratch builder to make them look good. I have started to dabble in them, and the beginnings are promising.
So here is what I have on my second attempt. What I have done is to use a saw to cut off the front cowl and the forks. Origonally I just had the new forks (made from plasticard) sticking out from the bottom of the piece that has the twin-linked bolters, but this looked weird. It was too thick to too thin. I tried adding additional pieces of tubing of different thickness to try to taper it off, but nothing seemed to work.
Until I looked at the fender. What I've got glued under the twin-linked bolters is the front fender hacked up. I think this makes it look much better. I used a pin-vice and then a dremel to widen the holes. Then I stuck the tubes through and glued them to the main part of the bike. The end of the forks are rough yet, and will eventually have to make the flush with what will become the front skis.
Here is a concept of what the back will look like. This is just a first attempt and it is not attached at the moment. I have a better looking idea going on in my head, but this is the basic idea. What you see is a strip of plasticard glued on to two paperclips that have been cut off to make an almost triangle shape. I have put two bits of tube at the front and back of the paperclips to give it some, "working parts," and I have cut out two bars from sprue that attach the back end of the snowmobile to the back end of the track (sorta like a swing arm on a motor cycle). The problem with the current incarnation is that the paper clips are cut two small and the plasticard is cut too small.
The second attempt which I am currently working on will be larger. Then I will cut out some small strips of plastic card and add some detail to the tracks.
The biggest worry I have at the moment is the front skis. I want them to curve up at the end as well as taper off like real skis do, and I am not sure how I'm going to get that to work with plasticard. One thought is to make sure that the base has a small stone right where I want the bend in the skis and then glue the skis down and let the stone push the ends of the skis up. But I'll cross that bridge when I get to it.