If your headstock is newer than a 1962, you have the larger motor. Or if you bought your machine used, check the motor because some owners did change out their motors over the years.

Unless your motor is fried, I typically tell people to keep using their existing motor, whether it is a 3/4 hp or 1-1/8 hp. The original Shopsmiths, the 10e and 10er, did and continue to do fine with only 1/2 hp motors. A headstock with a 3/4 hp motor should do anything you want it to on a Mark V. As long as your bearings in the motor are good, it should give you great service. Changing bearings in a motor is a lot cheaper than buying even a used motor.

I have also read than the standards for measuring motors changed about the time Shopsmith went to a 1-1/8 hp motor, and that the new 1-1/8 rating is the same as the old 3/4 hp rating, so they may in fact be identical in energy output.

The motor used in Mark V's is unique. You can't just take any old off the shelf motor and put it in a Mark V. The floating sheave system for speed control requires a much longer shaft than is typically found on motors. Thus you either need to buy a used one (which may need bearings itself) or a new one from SS. Either costs you much more than taking care of the one in your machine.

About the only user who could benefit from a 1-1/8 hp motor over the 3/4 is one who owns the SS Mark V mounted planer (not jointer, planer). The 3/4 hp is fine for drilling, boring, sanding, lathework, sawing, and all the other SPT's and add-ons made.

I also believe the 3/4 hp motors are very well made, better than those from the 1980's on certainly.

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Should I upgrade to the Poly V drive system?

In many ways the Gilmer system is superior to the Poly V system so if your headstock has a gilmer drive, I would keep it that way.

The quill and drive sleeve contact securely metal to metal with the Gilmer. The quill and drive sleeve meet with a sacrificial plastic drive ring assembly with a Poly V drive.

The "tank treads" of the gilmer belt are less likely to slip, although most have a force-activated clutch in the drive sleeve to allow for slippage.

Gilmer belts don't develop a distortion from years of storage and they don't wear out as quickly as the poly v style. Poly v's can become distorted easily from sitting and have to be replaced for that reason alone.

Poly v's also require more tension to prevent accidental slippage and that is harder on the drive sleeve bearings and the idler shaft bearing.

To convert from gilmer to poly v requires changing your drive sleeve and your fixed idler sheave along with the belt, adding to the expense of the conversion.