Are any other parts of your car modified? Chassis, suspension?
The adaptor plate is likely to have come from Banks (Europa Engineering) and maybe the NG3 too. Does it have the Banks gear change mech from the stick through to the rear?
Sounds like your options are all none original so far.... does it matter to you?
If it does then you can build a pretty good Renault engine and have a very original car - I assume it’ll be at least a second car so you won’t be expecting to put thousands of miles a year on it anyway?!
If you aren’t that concerned about originality then the best thing to do is build it using whatever interests you! - it’s your car and you are going to be driving it and hopefully enjoying it. - however wild you go it is always possible to revert back to original if they end up being worth millions in the future!
Richard @ Banks used to use MBE ecu’s and the red top was quite popular at one point, I’m sure the manifolds or throttle bodies are still readily available along with any other parts required to spice it up a bit (more than 200bhp) and the wiring harness, exhaust etc required to run either of your Vauxhall engines would be something that I’m sure he could assist you with. - he even had a plug cover insert for the red top that deletes the Vauxhall branding and replaces it with ‘Europa Engineering’.
You’re likely to need the water pump front cover altering for the twin cam as well, it’s different on a Europa vs Elan I think.
There are a lot of other mods available that could compliment one of the engines you have by further modernising other parts of your car - depends what you intend to do with it really as to how far you go?
Whatever you do, post some pictures for everyone to look at!