You don't really say what you're trying to do. As the question hasn't been answered for a couple of days, I'll have a go at responding, although, I've not done a IRIX install or root password reset for a fair few years now.)
I assume by "no root password" you mean "there is a root password, but I don't know it". (Fresh from an install IRIX has no root password, and you set it as part of configuring the machine.) and that you'd like be able to to use the machine.)
Your options are either to do a reinstall, or to boot the machine and delete the password hash from /etc/passwd (probably with vi). If you had a working SGI, then you could connect it to that machine. Linux's support for XFS may also work to edit /etc/passwd, assuming you had a PC with a SCSI card.
If you need to do it from install media, then the "Installation Tools Disc" is the first CD of an overlay set; the images for 6.5.22m (m for maintenance branch) should be easy to find, as SGI made these available for download without charge. (I am not sure if you'd be better using 6.3 media to reset the password on 6.3 install.)
https://software.majix.org/irix/admin-password.shtml seems to be a reasonable guide.
If you're doing a reinstall, then you'd probably move to 6.5.22m (or the final version, 6.5.30). 6.3 is probably a little faster, but finding software (particularily ports of open source/free software) will be somewhat more difficult. The nice thing about 6.5.22m was that SGI put the overlay CDs up for download, so they are very easily available. (However, since the death of SGI, 6.5.30 has been very easy to find!) As well as the overlays (which is about 4 CDs) you need the Foundation discs (1 and 2), and the Applications disc, and any other discs (NFS?) you might like to add.
I've used Ian Mapleson's guide with success:
http://www.sgidepot.co.uk/6.5inst.html (there's also a variant for network installs on that site).
Regarding whether you can write them; people clearly do manage it. The only tricky part, perhaps, is that from memory, they use a 512 byte sector size. I'm afraid I've not done that personally, I've been fortunate enough to collect official CD sets. An alternative to CD boot is booting from a network, there are largely preconfigured VMs for doing this (I believe along the lines of "just add CD images").
My experience of booting an SGI from install media has been limited to directly from official SGI CD media to install an O2, and netbooting an Octane to clear the root password.
(I've chosen not to link to IRIX images; I think the admins would prefer I didn't. It shouldn't pose much issue as images seem very easy to find with a web search.)
