To start with, let's presume that you have a well-running, easy starting Virago or Chain-drive that is a pretty fun ride. But you'd like a little more. You might:
Drop in a C05 cam set (in under an hour) and - if necessary - put a washer under each jet needle clip at your next tune up.
The entire series can benefit from a carb swap: as little as VM34s for low-end power, up to a pair of 42mm HSR for a large displacement screamer.

Removing A Flow Blockage.
Remove The Aluminum Edge.
Put a grinding head in your drill and open up the intake ports to match the ID of the rubber carb flanges. Be neat, but don't obcess: a 240-280 grit finish is good enough althogh you could strive for 300. No finer than 320: see the topic "Laminar Flow" on the information site..
Broaden and polish the exhaust port until you can shave with it. Don't raise its roof: you will hit oil and ruin your heads.
A lot of old accessory pipes are available. Be careful - although the engines are all shaped the same - the frames are not. Be prepared to modify. Most acessory exhausts are quite short; remember that long exhausts enhance low-end power. Have some made. And try to keep it quiet
Under NO circumstances run a single carb manifold. It will void any warranty on parts you get from me. Sell or destroy the bike rather than submit it to such abuse.
What if you have an old non-runner out in the garage? Its starter mechanism is probably shot and the rings gummed shut.
Gen I bikes usually trashed Idler Gear #2, which can simply be flipped over to double its life. If both sides are worn, it is still available for about $125
See elsewhere on this site and at this page about revivifying a weak starter system.

All the oil pumps interchange (including the V Star). Pull yours and disassemble it. With a sheet of 300 grit sandpaper on a sheet of glass, with a rotating motiom remove all scrathes on its (internal) surfaces. Now take some 400 grit and try for a mirror finish. Clean thouroughly; reasemble. I like to use ATF everywhere. For the overhead oilers, widen all oil holes as much as you dare (the bolts are _very_ hard and brittle); champfer the holes. Run high quality 6205 bearings in places of those damn aluminum cam bushes. They whir a little. Make sure you are using bearings meant to run in hot oil. Do NOT run wheel bearings with the seals popped out. With cam kits, I include SKF, NOC, or similar.

The oil pressure is very low, which is odd considering these engines run babbit (insert) bearings for the rods. An oil pressure guage mounted on the stack on the case where the upper-end feeds are will oftem show zero pressure at idle. The hotter the bike is, the higher it must be revved to move the needle. Regardless of what your manual says, I think that the oil pressure relief is full open at 15PSI. Remember that these are wet sump engines and most lubriction is by splash or oil whipped up top by the cam chain. Make sure the overhead feed lines are open. Early 920Rs tended to sieze their front cams. Re-drilling the internal feeds cleared this up. It required a warranty head swap. There may be some advantage in supplyng more oil to the spinning and reciprocationg parts, but don't get cute with pressure, I have never seen an oil pressure related failure of any sort - except when someone put the oil filter in backward.

The stock valve springs (common to every model) tend to bounce in excess of 8000RPM and coil-bind at .440" - this is covered under cams. Stock TCIs (spark-makers) cut off below 9000RPM. C01 and C06 cams are just stating to pull strong at this figure. A safe redline is in excess of 10500RPM - these bikes should pull like Desmo Ducatis or - shudder - Vincents. For a lot less than a new bike they can be nade into eleven second quarter-milers. To accomplish this you must acquire an Ignitech ignition, There are other gimmicky, under-the-cam-cover ignitions. They are very neat, but to get the flexibility of a $350 TCP-4 Ignitech, would - after all the add-ons - bring them up near $1000.

Due to the layout and internal measurements and parts interchangeability between the various models it is possible to start with a 700 Virago and build it into a 1631cc stump-puller. Personally, I feel that if you are looking for real power, to start with a chain-drive 920 (or a TR1) as you will almost certainly wish to be able to change the final gearing from time to time. I have seen shaft-to-chain conversions, but they are costly in labor, engineering time, actual fabrication of the parts, and their fitting to the bike. If you want to change the ratios between the transmission speeds you can find complete transmissions cheap. Sadly, the hard parts sites usually don't list tooth-counts; so you may need to buy stuff you don't end up using. When you change a gear on one shaft always always change to its mate on the other shaft! If the teeth don't mesh thoroughly, disaster lurks at your first shift to that gear.

Also, if you are upping the power considerably you will want to fit the vehicle with more modern running gear: "upside-down" forks, four (at least) live piston dual calipers out front, a longer and wider swing arm, a good shock.

Some more specific suggestions; added to as I have time: