If you choose to use K5 forks, it will be a matter of changing the forks and triple clamps, stem should be the same. 88 I think had a smaller front axle (not sure about that) if so your going to have to change your bearings to the newer dimensions of 20 mm. as for brakes I'm not sure.
If you want to mount 250f or 450f forks it becomes a bit more complicated. I mounted a set of 450 triples last night on a 500 chassis, what I found was the stem is to long, no problem press out the stem and away you go, wrong. The 500 stem on the big end is 30mm, the 450 is 30.19mm, I was rumaging through some of my spares and found a stem that was 30.19 on the big end and all the other dimensions were the same as the 500. The stem was stell and I think it came off a older 250 or 500, anyway it fit perfectly. The bearing area is 30 mm same as the 500. on the top I had to make a bushing for the top clamp the stem bore was 25mm and the 500 stem is 23.46mm anyway about an hour later and they wer installed. I plan on running a 450f front wheel so no big deal there, and the brake was already converted to 250f so no issue there either.
All in all about an hours job with the right tools.
This frame was a 2000 500 and the forks used the 500 externals with 2001 250 internals, they worked quite good so that may be another alternative to getting better forks.
Many have said get them revalved, but honestly there is only so much you can do with the stock forks, the newer stuff is quite noticeably better, and of courrse it depends on what your going to be doing to.
