If you have the hollow spindle in your monster front wheel, the 620 sport ones will fit straight in.
If its the solid spindle, then you will have to change the bearings and spacer (could cost up to £50+/- in bits)
(Hollow - big, like all later bikes, and solid - smaller diameter spindle)
Will very much depend on the front forks you have in the bike as to what spindle it has, the later (98 - 01) carbed monsters had a mixture, so this is very very confusing!!
Usually, showa's had hollow and Marzocci had solid - but this is not always the case!! (as I have found out!!)
Rear wheel from your 620sport - definately wont fit a 99 monster, unless you change the bearings and spacers. There are more than one size of rear wheel bearing diameter and spacer tho........
It might be cheaper to pick up replacements if you need to start doing bearings (unless you are doing the work yourself) and flog your wheels to someone else.
Hope this is of help Tonio,
Alan
[Edited on 21-11-2005 by AK and CK]