Assuming your transom to be varnished rather than painted the problem in choosing the wood is largely cosmetic,  rather than structural;   any good marine hardwood would be structurally suitable,  but the difficulty is obtaining a good cosmetic match,  and since the existing wood is very likely to have faded you will be trying to match new wood to old faded wood.   So the best visual match may not necessarily be the same species as the original.

A further complication is that the new wood will fade more rapidly than the continuing further fading of the old wood.

One option would be to consult the catalogues of specialist marine timber suppliers,  such as Robbins or Stones or Sykes,  and see what looks closest in appearance.  https://www.robbins.co.uk ,   http://www.stonesmarinetimber.com ,  https://www.sykestimber.co.uk/index.html .

On older GP14s than yours I would expect the timber to be mahogany,  which I fully realise is a family of timbers rather than a single species (around 400 of them,  if my recollection of my O Level woodwork course well over 60 years ago is correct).   For your boat,  good starting points might be African or Brazilian mahogany,  sapele,  or utile;   or just possibly iroko,  or even meranti.   Mahogany and iroko are traditional boatbuilding timbers,   and sapele and utile are also sometimes used (but are slightly less durable than the other two);   however meranti is not a traditional boatbuilding material,  although it is a reasonable quality domestic timber and you might get away with using it if it happens to be the best match visually.

Alternatively,  if you know who built your boat (and if the firm are still trading) they might be able to supply you with a suitable offcut of original material.

Hope this is of some help,

Oliver