what "spings" to mind, and I won't keep you in suspension, is that I use 20w fork oil and fill it 6" to the top of the tube. (forks fully compressed with the spings removed)
The motor oil option performs badly as i belive that the oil foams and thus reduces it damping properties.
wurth (german) and hagon (british) do an uprated progressive sping option if you want less dive.
a 400 front end will fit yours if you want disks, but offer out the drum front end as lots of peeps are after those to do cafe racer/bobber bikes!! don't forget that you had a spoked front wheel and, unless you can get a 250 spoked front hub, you will have a front alloy wheel!!
hope I didn't put a "damper" on things!!