So far as the valve spring compressor is concerned, my experience suggests that it is essential to use one for cars, because they have high speed engines with big valves which therefore require very strong valve springs, to prevent valve bounce. They also use split collet valve retainers rather than the keyhole retainers more common on low speed engines. When it comes to small mower engines, finger pressure is normally sufficient for the keyhole retainers they mainly use. When you get to large mower engines, say above 200 cc, you are in no-man's-land: you might get the job done with fingers, or you might end up with blood-blisters and engine parts flung violently into the distance by escaping springs. You have to make a choice: spend $40-50 on something you may never use again; or work slowly and carefully in a clean area (so you can find the bits that get ejected violently) using the spacers-and-screwdrivers approach; or make up a forked lever to use with a flat plate pivot screwed to the front of the engine.

Your main problem is that you are currently working on side valve engines, and generally, most of the available valve spring compressors will not work because the spring retainer end will not fit into the valve chest. This picture shows the type I used long ago, with a fair amount of cursing, until I finally threw it away and restricted my attentions to mower engines:
[Linked Image]

It can work on most kinds of engines - the main problem is that it is rather springy and not well suited to car engines with strong springs; it has a habit of twisting at the critical moment. I once had a massive cast iron one that worked on side valve engines only, but I threw it away too: side valve engines too large for finger pressure have no appeal to me.

So, do you have keyhole retainers or split collet retainers? For keyhole retainers, I'd try the fingers method. For split collets, I'd probably make up a forked lever, because it would probably be fun, and I'm a bit tired of hurting myself and losing the flying parts.

I'll send you a PM about the G400 manual.