You could use silicone if the visor was mounted on the exterior of the mask, otherwise you could have the lense blown back into your face. Also, you shouldnt use a motorcycle visor, as it isnt rated for the same impacts as paintball ones. All the feild owners i know simply will not allow anyone without proper face protection play, and for the sake of the sport of paintball, i agree.
you can find many types of lenses at www.actionvillage.com
which will probably be the cheapest unless they are on sale at www.firstcallpaintball.com
They have alot of different tints, although they may be around the same as a motorcycle visor. The only problem i can see is the lenses for paintball generally arch upward toward the nose, although that could be incorperated into the black nosepeice on the mark VI. I'd suggest using the Grillz lenses, as they are a compound curve and can actually stop small caliber bullets. However, you may find the JT spectra lenses in a wider variety, and i think that they are a bit larger. You may have problems though because almost all tinted lenses, being higher quality, are thermal. Thermal means it has a thicker outer lense and then a smaller, inner lense. This design helps to dissapate the heat you generate and greatly reduces fog, however they are led together by an airtight seal between the two. If you trimmed this lense, you may have to lose the inner lense because once you break the seal the tharmal abilities of the lense are gone. With a fan to suck out the heat,though, I doubt you would have any problems.