sierra's over here have beeen using roller followers without issues, however there would be a lot of work involved in either getting a custom cam made to line up with the centre of the follower guides or machining th ehead to get them in the right palce, let alone lubricating properly.
it may be better to try and open out a 1.8cvh sierra head and seeing if that could be used to get the power out and better cam life/reduced noise etc.