How you electrify the Spirit depends on what you goals are.

If you want the best thermalling plane but would like to e-launch from time to time, get the power pod. It is $32 from Tower Hobbies:
GP PowerPod

I don't see any reason why you could not modify the powerpod to make it taller so you could put on a bigger prop, or gear it and put on a deeper pitch prop.

By the way, it now comes with a speed 600 motor. I just took delivery of one. Haven't finished assembling it yet.

If you have decided to abandon hi-start and winch launches totally, then I would suggest you mount the motor in the nose. Can't recommend what you would need, but you could take a look at the manual for a Great Planes Spectra kit since it is so similar to the Spirit. See how they handle the motor mount there.

Spectra manual

A third approach would be to get a Great Planes Spectra kit. This plane is virtually identical to the Spirit, but it is powered. You can buy the whole kit for $69. Build the fuselage and swap fuse while keeping your spirit wings. Pick which fuse to use depending on when you want to soar, or you want to e-launch. This way each fuselage is properly balanced for the way it will be flown. And the rest of the kit serves as spare parts.

Just an idea!

I bought the powerpod because I am really most interested in thermaling. I have two other electric planes when I want to fly electric. I want my spirit as a true sailplane.

All depends on what your goal is.