You had best get the original spring. Get both springs while you are replacing springs.

The main problem in working on the spring is holding the valve up (or closed) while you are getting the retainer off and on.

To do it with the engine assembled, you will need to have a glow plug adapter to fill the combustion chamber with air pressure or else stuff enough string into the glow plug hole to hold the valves up.

The other way is to remove the head from the engine and set it over a little wood block or something to hold the valves closed

When you have set up some device to hold the valves close, you can push down on the spring retainer and slide out the valve retainer keys.

Then the retainer and spring should lift off. If the retainer will not come off with both keys removed, there will be a lip on the edge of the valve stem that has been peened by the keys or the rocker arm. This lip will have to be removed with a fine honing stone before you can get the retainer off the valve.

Another problem with this job is that those little keys will go flying through the air and you will loose them. You can do all this inside a plastic bag so if the key goes flying - It will stay in the bag and you can find it.

Either way, the rocker arms will have to be removed and you will have the reset the valve lash when you get the engine reassembled. The valve lash setting is .002" - .004"