Bernard,

I beleive the Black Horse Edge 540 .60-.75 is similar to the Seagullplane you have.
the DLE20 is too large for the BH version because the engine needs to be mounted with the prophub 115mm from the firewall. the DLE is 135mm long...

Clive McInnes of CMC hobbies will have the NGH 17cc shortly. Its shorter then the DLE because its carb is in front al glow style and will fit the Edge. Its not much lighter than the DLE but my BH needs a bit of nose weight even with a OS 91FX on the nose. With a 2stroke glow these ARF's need the battery as far forward as possible. the NGH may be the ticket.