Here is a link to MSC any of these reamers should work. Your ~measurement you gave and the numbers the service manual that was given by torch say you need a .275 to .2762 reamer.
it's a simple process and can be done by hand with a hand reamer. But is easier with a drill press or a vertical mill.
https://www.mscdirect.com/browse/tn...4+4288235420+4287908817+4288140076+4288247102
If the link does not work go to hole making/reamers/chucking reamers & sets/ chucking reamers. then filter them down by decimal inch to any sizes in between the numbers I gave or that you calculate. also filter them by high speed steel and straight shank type (no need for carbide unless you have some cash to waste). At a minimum you need a drill press do not ream with a hand drill or you will be asking for a out of round hole.
Since you need to get a reamer order some tap magic aluminum cutting fluid and use it while reaming it's for cutting in non ferrous materials
https://www.mscdirect.com/product/details/00262337
If you don't have a drill press then use a hand reamer with one of the ones from the link below
https://www.mscdirect.com/browse/tn...12106147#navid=12106147+4287690171+4287908392
its a hand reamer and uses a tap handle to drive it this will take a little longer but is much better than putting a reamer in a hand drill and messing up the valve guide. if you already don't have a tap handle you can order one or your local big box hardware store will have one or anywhere that sells taps you don't need a expensive one
This brings me to my last point the reaming drill speed should be about 600 rpm for that size of reamer in bronze (i am guessing that is the valve guide material). The actual rpm calculation says you can go slower or higher but speed is not needed for what you are doing. if reaming by hand keep the reamer straight.
Youtube is also a great place for information on how to ream valve guides.
I will agree with Pau7220 the correct way is in a machine shop. I say if you want to try it yourself go for it worst thing that happens is you have to replace the guides again.