I had the same concerns 18 months ago when I replaced my 70mm Marzocchi Z4 with a 100mm Magura Vidar on my 1999 Kona Caldera
(http://www.retrobike.co.uk/forum/viewto ... highlight=
Whilst the steering is a little more lax it took very little time to get used to. I contacted Kona direct and they advised that by increasing fork travel by 1 inch it slackens the head angle by 1 degree (I think I have that right!)
When looking for forks though axle to crown lengths varied wildly. E.g.
80mm Marzocchi almost equates to my 100mm Vidar.
I think axle to crown on your bike will be 420mm.
420mm = around 70mm
440mm = around 80mm
470mm = around 100mm
When my Vidar is locked out e.g. at a full 100mm, it does feel stranger than when active and sagged but this will be the case with any fork.
You could try some suspension corrected P2â€™s (470mm axle to crown) this would at least give you an idea of what a 100mm fork would feel like but on the cheap.
However, despite being cheap it can be misleading as 470mm axle to crown is roughly an uncompressed 100mm suspension fork â€“ which unless ridden locked out you wouldnâ€™t notice.
Hope than helps!