For the valve springs: maybe put up a request in the Parts Wanted section in these forums. While the OEM F2/3 springs work, they are excessivley strong and can wear the cam, rockers, and valve keepers over time. I haven't seen an F2/3 make it past 45k miles with those springs unless it was rebuilt. In contrast, the K7/8 springs will give you a solid 80K mile top end if you're not racing.
The cam chain roller: yeah, what Ekpent said: the OEM version is about $60, the bad ones are about $30.
The most important item you can get is Ken's valve spring retainers, about $130 for the set. Next most important is better valve guides, as the OEM versions seldom make it past 10k miles. The machine shop I use here charges $15 per guide for sizing and $12 per seat to recut the seats to match the new, installed, sized guides perfectly - I charge about $50 to pull the old ones and install the new ones. It's about $7 per valve to then have their faces cleaned and cut to match the size of the seats this made. Generally these head rebuilds run around $500-$600 from my local shop, plus the new retainers and the rubber pucks. You can get the rubber pucks from places like PartsNmore or CB750 Supply, about $4-$6 each one. Be careful to get the ones for the CB750: there are others out there that look the same, but are thinner and WILL leak when you put it all together. This caused a lot of confusion and leaky 750/550/500 engines in 2005-2010 when the thin ones were coming in the gasket kits by mistake. Those thinner ones fit later bikes, like from the early 2000 era.
If you have a local machine shop, take My CB750 Book with you to the shop and show them the valve stem clearances you want, and the piston clearances you must have. If they balk at them (because they are small clearances), find another shop. Some shops think they know better: don't believe it, or them. The rebuild WILL fail immediately if these tolerances are not kept.
Shipping back & forth to me would probably break your budget: it can cost $80 to send a head across the USA one way today.
Boring the cylinders: same rules apply, take the book and show it to them. If they don't believe it, tell them to e-mail me and I'll explain it to them - as civil-ly as I can - because normal engines (water cooled) use lots more clearance and these engines will fail unless kept [very] tight. I even rebuilt my own 750 with the old Honda racing spec of 0.0006" piston clearance, and the reward for having done it was immediate, and terrific! Mine has never run so strong nor so quiet before. The local shop here charges $35 per hole for boring and $55 for putting it thru their parts washer and bead blaster, $45 to mill off the 0.010" to compensate for the new head gaskets being too thick today.
