Replacement engines are plentiful, and if you're willing to tidy them up and replace a few parts with good ones from your old engine, can be very cheap:
https://www.ebay.co.uk/itm/153060221852
https://www.ebay.co.uk/itm/192216995395
Looks a big job to replace the engine but its not actually that bad once you get stuck in. Just like a big lego kit, take loads of pics while youre taking it apart and then reverse the procedure. You will need a manual and a couple of torque wrenches. While a £3k rebuild would give you a nice fresh new engine, not everyone can afford to go down that route. Luckily for those of us that are often skint, there are alternatives
* edit * guessing the SE is a twin spark, in which case you may be a bit more, worth calling sherlocks to find out though.