Gravity boiler to forced water conversion

Biggest one is the mass of it(the water, the pipes, usually the cast iron radiation), if you use a conventional boiler you will need some sort of return water temp protection to keep the boiler from sustained condensing(or use a mod con boiler).

Radiators may have orifice plates in the valve unions on the upper floors if it is more than one level to slow down the flow to those radiators. You will either have to remove them or move them to the 1st floor if they exist. Gravity circulation favored flow at the highest points, pumped circulation favors flow near the boiler.

You would do a heat loss of the house and size the piping and circulator and boiler off of the heat loss, the piping to the boiler will be much smaller than the gravity supply and return mains.

Probably will need to use the radiator valves to balance it.

If it is an open expansion tank convert it to a bladder tank, if it is a compression tank you can either convert it to a bladder tank or arrange the near boiler piping to separate the air and put it in the compression tank.

Use a magnetic dirt separator to protect the wet rotor pump from the 70 years of much that will get stirred up by the circulator.

Not a bad idea to run some cleaner in it and flush out as much muck at possible before/during/right after the install.

We have two gravity conversion systems heating our old 4800 sq ft condo building near Boston. One comment I would add is that gravity conversion systems typically have very low head because the pipes are so large, so you typically don't need a large circulator unless you have a big boiler like the one @EdTheHeaterMan has in his photo. Our building is split down the middle with two identical systems each heating 2400 sq ft through 10 cast iron radiators, and a Taco 007 on each system that gives us about 17 gpm, which is plenty for our 168,000 BTU/hr oil input on each boiler.

We also don't have bypasses on our boilers (which Ed correctly says are required by the boiler mfr, but our installers neglected). The bypass will reduce the flow rate out to the system, but even if we did have bypasses, assuming half the flow goes through the bypass and half into the system (for example), we'd be getting 8+ gpm through the system, which would probably still be enough for us. I have a variable speed control on the 007 and have experimented with dialing it down to 10 gpm, and there's no difference in system performance, other than a higher delta T across the boiler. But it still heats evenly and takes the same time to satisfy the thermostat at the 10 gpm flow rate.

You might find this article on circulator sizing by @Steamhead useful. He says to size assuming 3-1/2 feet of head for gravity conversion systems. By back-calculating based on delta T across our boilers, it looks like both our gravity conversion systems have about 4 feet of head, based on the pump curve for the Taco 007, which is very close to the recommended design point of 3-1/2 feet of head.

https://www.heatinghelp.com/systems-help-center/sizing-circulators-for-old-gravity-hot-water-heating-systems/

We have about 500 sq ft EDR per system, which according to the chart in the above link should have about 12 gpm, but as I mentioned, I've experimented down to 10 gpm with no apparent problems.

The chart recommends higher gpms based on our boiler BTU rating. Depending on whether I used the 168 MBH input rate or the 138 MBH output rate (at 82% efficiency), the chart recommends 20-25 gpm, which is overkill, as I've found that 10-17 gpm works fine for us.