I would not buy them sight-unseen without inspecting them first for any flaking chrome or pits. If there's any deterioration on where the fork seals 'ride,' then 'just say, no!'
Do it, no issues. I believe the later model at some point went to a bit shorter axle in the rear, but I'm pretty sure the front is the same. You need the newer calipers and rotors too (see below), unless you're using some type of bracket for mounting the calipers now. I believe the 1985-'89 calipers are (I believe) 82 mm o.c. and the 1990-'07 are 100 mm o.c. Check your rotors, are they 298 mm diameter? That's what the 1990-'07 OEM rotors are. I don't know what your Brembos are for caliper mounting distance, or the rotor diameter. Do they mount directly to your 1986 sliders, with no mounting brackets?
You mention
"full 02 front end" (tire,disc´s,axel and clanps [sic-clamps]) but no mention of fork downtubes or sliders, I assume that's just not mentioned, and they are part of the package, along with the fender mount/fork brace. You will get greater stiffness in the front end from the greater diameter. I'm not a mechanical engineer but I believe the greater stiffness is more-than the simple increase in diameter would seem to provide (43 mm being 7-1/2% greater than 40 mm).
Look at some aftermarket springs if the front end doesn't have already come with them, who knows, you may luck-out and they're already installed. Be sure to change the oil. If you choose to replace the fork seals while things are apart, OEM seals provide great service life.
Since things are going to be apart, check the front end steering head bearings for sufficient grease, any brinelling (notchiness of the bearings from loose bearings and/or too-many high-impact wheelies). Follow the proper factory manual sequence for tightening the front end axle, triple tree pinch bolts, steering stem acorn nut and double castellated nuts, and the fork brace/fender mount.
VMX12- Service-Manual.pdf (vmoa.net)
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