Here's a motor that should do it for $199.95.
5 HP 230 VAC 3450 RPM LEESON AIR COMPRESSOR MOTOR
Here are the lovejoys. The motor above has a 5/8 shaft.
5/8" L-099 JAW COUPLING HALF
If your pump has a 1/2" shaft, you will need this other half.
1/2" L-099 JAW COUPLING HALF
And this rubber spider.
BUNA-N INSERT FOR L-099 JAW COUPLING
You will have to make a "L" bracket to support that motor vertically, since it mounts on the bottom.
Don't forget you will need a pretty big switch to turn this thing on and off. Surplus didn't have anything that would work besides a relay.
I just took an old gas lawn mower and made a splitter drive. Learned a couple of things:
1. My pump looks like yours (16 gpm at 3600 rpm), is 2 stage and cannot tolerate lateral stresses. I like "ajmaxes" installation as it solves that problem of lateral stress. The pump shaft has to turn CW on mine and maybe yours too. I like the motor Franklin 2 picked out as it can be reversible....to drive the pump in the desired direction, is a high torque application.... to hang in there when the splitter loads up and kicks in it's high pressure function. The 3450 rpm matches the rated pump capacity of mine and supports the direct drive like aj has in his installation. Otherwise the drive gets complicated....V belts, different sheave diameters, additional brackets, selection of the correct belt to carry the HP. My pump has no position limits; can be mounted in any position....vert, horiz, 45 degrees.
I encountered other problems that required funding but you aren't facing the same peculiarities in your electrical application.
I bought my supplies at Northern Tool. I did learn that their Lovejoy couplings listed in their catalog for $20 are just one half. So you need to buy two halves and the rubber cushion Franklin mentioned. That makes it easy in selecting one shaft size for the pump and a different one for the drive once you realize you will be spending $40-50 for the hookup. The Couplings use a square key to lock them to the shafts. You can get key stock at an auto parts store and cut to the desired length. I left a little gap between halves to prevent any stress transfer that wasn't part of turning the pump.
Wattage to hp correlation is 741 watts (volts x amps x power factor) per hp. Power factor is around 1 or less depending on motor design and load. Since hp is (torque x rpm)/5250 or thereabouts, with the motor and gas engine running at the same rpm, the only difference is torque. On the difference in electrical hp to gas hp, the difference has to be in the way the motor is wound and started to get high torque. Cap start with a series or series combo gets you the high torque you need to handle the load. Again, Franklin's motor selection looks perfect. Gas engines just don't put out a lot of torque for that size. I guess that's why they started torque rating them rather than hp rating like has been done in the past.
On a switch, I don't see a problem at all. 15 amps is not all that much. A cheap switch is a 15 or 20 amp 240 volt single phase circuit breaker like you would have in your power panel for your house, running a small air conditioner. Get you a little NEMA box to put it in, wire it with suitable cable and go for it. If it wears out, it will take awhile, and replacing is no big deal.
The starting load on the switch is minimal as the pump is not under any stress to speak of so the switching current would be low; basically a little startup spike and the idle current of the motor which is nothing as compared to the full load current. Same when you turn it off, no load means low current, means a small arc upon breaking and little contact wear. The big load comes when the ram drives the log onto the spike, in the first instant, and the switch is closed and could care less.
I had fun building mine, works great and would do it again in a heartbeat.
Good luck,
Mark
