My advice is not to start!
For the effort involved in converting, you are not going to end up with a good representation of a Terrier.
I tried this many years ago. A major issue is that the old Dapol/Hornby chassis makes the whole body sit too high and the loco does not look nearly as diminuitive as it should be. I resorted to a Branchlines chassis and (much to my amazement) managed to make it work with split axles and frame plus sprung front axles. Having invested in a decent chassis and wheels, I then discovered the hard way (much carving and cussing!) that the old Terrier body needs a helluva lot of work to bring it up to the same sort of standard. For starters the body is a sort of hodgepodge betwixt an A1 and an A1X….. I gave up after filling the smokebox with lead and a month later the whole boiler became visibly distorted….
After a long period (years!) of indecision and procrastination, along comes the new Hornby Terrier which I have converted; it looks good but it is not a roaring success chassis-wise; low-speed performance is not good. I might fit the old Branchlines chassis but that raises the issue of how to fill the small thing with enough weight to improve the road-holding; this will also be an issue if you persist with the old body and build a new chassis for it. My solution might be to buy the old Albion brass kit, now available from Roxey, and put a Branchlines chassis under it; at least the metal body adds much-needed weight. However, the Terrier is at the bottom of the Round Tuit drawer…..I resorted to a B4 and a Midland 1F as alternative small shunters.
Good Luck! You might need it!