can someone give me some advice
My bike had a replacement engine less than 2k before i bought it , The dealer never informed me and i have only now found out as its int he back of the service book , The engine number no longer matches the one on the V5 , Should i be worried and should i of been told before i purchased by the dealer ?
In my opinion, it's a paperwork issue.
But there are a couple of potential 'get out clauses'
A) If you purchased the motorcycle remotely over the telephone or online, and never visited the premises to view the motorcycle, you are covered under the Consumer Contracts Regulations 2014 (the old Distance Selling Regulations) and can reject the deal up to 7 days after you received the goods (or until they notify you of the cancellation procedure, this can extend your rejection by some time). Would this apply?
B) If you purchased the motorcycle on finance, then it might be worth speaking with the finance company to see if this situation would effect their 'valuation' of their asset. They might offer legal support to fight a reduction in price or refund, but that is a long shot.
C) If you purchased the motorcycle on Credit Card (any amount above £100) you could contact them under Section 75 and argue your case, see if they could pressure Dealer for a refund, but again that's a long shot.
BUT, having said all that if the paperwork had been completed on your V5 and you had never known about the engine change, would you still be happy with the motorcycle?
Is the engine and bike performing as you would expect for the money you paid?
If it is, then it's just your head playing mind games with you.
If you genuinely wouldn't have purchased the bike if you'd known about the engine change, then that's going to be a battle, especially if there is nothing wrong with the bike. They've not supplied you with a faulty machine. It's legal to replace faulty parts, and the argument could be if it mattered so much to you (you should have checked everything was in order before sale to 'your' standard).
It would bother me also, up until I realised nothing can really be done. Just get the paperwork changed, put it behind you and enjoy your bike.
Just not a nice surprise is it.

But with the BMW R1200GS and Adventure, Engine changes happen a lot with people having warranty claims over nothing more sinister than paint flaking! So I personally wouldn't worry to much in this instance.