If I can find a few parts Im hoping to rebuild my 29X RI RE.  First priority is finding new  a con rod. Engine was built by Nick Shur.   I have piston/ liner. Chromed liner in good shape, unsure about piston fit into the sleeve. Nick replaced stock Al piston with high Silicon "Vanasil".  unlikely I can find a piston, so might need to find resource to rechrome the liner and then rehone . I think chrome  work originally done by Gene Hempel. Final fitting by Bob Oge.  I might also need to find new cylinder head if I cant find the one Nick modified to accept a button. ( I have the button). last item I would  need to find is a pipe. Originally engine was setup for "Rossi B" pipe. 
Any help or guidance appreciated.
Lyle

http://sceptreflight.com/Model%20Engine%20Tests/ST%20X.29.html

Says 5 mm wrist pin , 6 mm crank pin . Which is pretty much ' V ' 60 ( stunt ) sizes .
Do you have a knackered rod , for the length ?

Ive just measured a G45 conrod , 5 mm & 6 mm  . AND the ' COMO ' , which is the same .
As the Como is a development of the G40 series , as is the X 40 ( & 45 & 29 ! ) it looks like
ANY late G 40/45/51 ( & 49 ) rod'd go in .

This is where it started ,
and this is the ' schneurle ' .

21 series theres long & short rods . ( dual and single ring piston ) you got that with the 1960s G 40 / 45 / 51 too . So theres ones that go in , and theres ones the right length , that go in .   

HooKay ; gotta Como 40 rod ( G 40 ) which is 34 mm center to center , as is the OS SF 46 rod , which is a 25305002 OS rod , for a " 46 AX , 40 - 46 FX . FXI .SF . VF Connecting Rod "
                                                                                                                                                                                                                 ------------------------------------------------------------------------
Theyre6 mm along the crank pin - Como ( Super Tigre )- 5.95 .  and the wrist pin ( both ). Whereas the OS rods 8 mm along the wrist pin . Gotta File .  as the OS rods a high output one , there ya go .
S Tigre's like OS . a rod of many colours , Er , FITS . Likely ANY  ST Rod from the original G series and Late schneurle G-40 to 51 will be interchangeable . Some rear intake rods ARNT ' round ' at the disc interface ,
on the 21 series - 21/40 RV frinstance .

I accidently put the S T rod in the SF last night . CLANG . the outer big ends a gnats dik broader . SO the OS rod'll clear nosweat . But maybe youd avta narrow the little end as the lands in the piston might be less than 8 mm apart .
Other'n that theyre identical bores and length between centers . A VF .46 rod , in fact . see Part No. above .

Lyle,

A local guy here has some NIP parts that, according to the ST catalog, fit the 29X, including connecting rods. I sent you a private message with his phone number so you can discuss what you need and see what he's got.

Dave

i will contact Patrick. If I recall, Dave Layman was involved with Nick Shur engine builds for the B record at Westover , Mass.