For a buget option you can fit any forks and front wheel as long as the travel is not longer than 220mm this including Marzochi forks from a R80/100GS. There is also WP USD forks availible with 220mm travel, might have to modify axel and brake disc to fit, or Touratech sell them to fit standard to the wheel of the R80/100GS front wheel and brake.
A lot of guys are right when they mention the lengthening of the swing arm by HPN when fitting long travel suspention, (more than 265mm) But on the main frame the steering haed area is reinforced as well as the fork rake angle is changed. This is a very specaliced job, and without this your bike will be very UNSTABLE at speed.
Check out HPN's site at www.hpn.de in english and feel free to e-mail Klause, it might take a while for him to respond but he is always willing to share information whether it means bringing in work or not.
If you have money to spend, HPN is the way to upgrade your bike, this will start with the reinforcing of the frame to adapt it to fit you suspension of chioce, 43 liter feul tank, fairing, as wel as reinforcing the subframe, lengtening the swingarm and centre stand.
Once you have done this and mounted the suspention you can rebuild the rest of the bike as is onto the new frame and modify and upgrade the rest of the bike with time.
I do not agree with the ST rider, a lower bike will have less travel and will bottom out mutch easier. One thing to consider as well is that more modern off-road forks employ a progresive method of regulating the oil flow. In short this means that the forks will adapt automatically to your riding style, speed and the road conditions. Older tipe shocks like those used on the ST or G/S have a small orifice that regulate this oil flow. It is obviose that this orifice will only allow oil flow at a specific rate, when the manufacturer decided what size the orifice must be they looked at the average use for the bike riden in European conditions and hence the forks will have hardly any performance for off-road use.
When building my HPN one of my greatest concerns was the ride high of the finished product, being very average hight I found that altough I am on tip toes when stopped this is no problem when you pay attention to the spot you want to stop, it needs to be firm and level and it is possible to stop with your right foot on the foot peg allowing your left leg more reach. This allow you imediate use of the rear brake if you have to let go of the handelbar. Afterward it is easy to shift your weight to the right side of the bike to get your left foot to the shifter. If you get the hang of this you can get the bike into neutral even before you come to a standstill.
Last but not least, if you dicide to add longer forks to your bike, do not do the frame modifications by yourself or anyone but HPN. These guys have been racing these bikes for over two decades, they use a large piece of equipment to striaghten and destress the frame after and before welding and they do this almost every day. By lengthening the suspention you place even more strain on the frame and it is very important that these areas are strengthened, I say this because I now of a no. of people that have done this themselfs and have seen photographs of some BIG DOG riders with so called HPN's of which the frame have never been to Germany.
Hope this is of some use
Altus / Cape Town

[This message has been edited by gsworkshop (edited 29 June 2005).]