Depends on what car you start with. Mate of mine will be doing a few of these conversions soon, he's got a Pearl Black 197, Albi 197, Albi 200 Cup and an Ultra Red 197.
If you buy a decent 197 for under £1500, sell the engine and box for about £900, by a cheap 225 for less than £1000 and you should have all the bits you need to do the conversion plus you'll have loads of breaker parts that will basically pay for all the parts to mod the 197. You just need to know what you're doing otherwise you get a right nail like some of them are where the wiring splicing looks like it's been done by a 3 year old
Just don't expect the engine to last ages, they blow up at standard power so ideally when it's out just stick some pistons and rods in it and you're laughing