Old Enya's use some very strange screw sizes. I think they are some japanese only size. I needed head screws for a enya .35III and never did find a correct replacement. In the end, I found someone selling replacement enya engine screw set for $5 and bought it to get this engine running again.

Unless you need to keep your enya original, you might just want to tap the exhaust holes to a common metric size, say 3.5 or 3mm.