Should I add venting to my solid soffit, and how?
Most roofs have intake and exhaust vents. Soffit vents can be used for intake, but they are not the only means of intake. If you have a vented roof and already have adequate intake venting, you don't need to add more. 

Additionally, not all roofs need to be vented - there are "hot roof" designs where the rafter bays are insulated and sealed up. You wouldn't want to add intake venting to a system set up like that. Given that your house is 100 years old, you probably don't have one of those types of roofs, but it's possible that it was renovated at some point.

So, figure out what you have and go accordingly. If you do decide to vent, you should take a look at all the products out there and install according to what you like best. For example, there are round vents you can install by cutting a hole out with a hole saw and pushing the vent in place, ones where you cut out a rectangle, ones where you cut out a long strip down the length of the roof, etc.

There are also vents that install under or in place of fascia board, which might be good for your situation (in place of soffit venting), since you say you're removing the fascia anyway.

I would not just drill holes - soffit vents are cheap and have screens to keep animals &amp; insects out.

This PDF has a good explanation of how roof venting works. They're selling Cor-A-Vent products, but the explanations are good regardless. They do make a good under-fascia vent, but GAF, Owens-Corning, and other roofing suppliers do as well.

Here are some GAF vents you can use for soffit venting. Others manufactures have similar products.