That's essentially what I had to do. When I was putting together my shift kit, I had to use the half-link for the middle chain (the one from the jackshaft to the chainring), but then one day when I was out riding, the half-link quit on me. So I had to replace it with a full link, but when I tightened it, the front of the engine was way off of the frame, since the whole engine slides up and down to tension that chain. I had a second (bad) engine sitting around that I was using for parts, so I grabbed the front mount plate from that and used it as a spacer. It looks like a dog's breakfast but it's holding up great.
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(yes, I know it's filthy, don't judge me)