£450 is a lot for an engine. a twinspark should cost £150 at scrapyard/breakers, & another £150 for fitting at local independant garage. make sure you get 28 days warranty on the engine. i would also do the full timing belt job & full service before fitting it, this will add £200 to the cost. if the car is only worth £1500 then you stand to make back £1000, you could sell it for £250 with a dead engine so you really only stand to make £750, which is a good enough reason to fix it imo, however that depends on the cost of repair. if you really cant get an engine for elss than £450, then i imagine you will also live in an area where other things cost more, so the money you stand to make back could be nothing. you need to do the maths and decide if its worth it, but you must do your best to get the best price. if you dont have a good relationship your local brakers and garages then that means ringing every scrapyard and breaker within 70miles of you, and then ringing every independant garare within 70miles of you, to make sure you get the best price. it may take a couple of hours but it'll save you loads of money.
Just a bit a lateral thinking - whilst I'm changing my engine, would there be any issues with putting in V6?
if you havent currently got a V6, then fitting one will be much more difficult and expensive than fitting a replacement engine that is the same as your current one. for a start you've got the wiring loom differences that will mean you need to modify your loom to suit the V6, which is labour intensive and oftren leads to electrical faults in the long run, plus the time it takes will double the labour cost. then you also have the issue of the ecu, which needs to change obviously, which means the immob codebox also needs to change, and then the keys. finding a matching set from a donor car is more difficult than you'd imagine, keys tend to go missing. you also then have the issue that your front suspension isnt the V6 spec to take the extra weight up front, and your insurance will cost more than for a standard V6. its a bad idea no matter which angle you look at it, unless you can do all the labour yourself. you'd be better of buying a V6 156, it would work out cheaper.