The MVVS 61, 77 and 91 are apparently able to be converted to rear exhaust just by screwing off the cylinder and rotating it.
I use a Webra .80 Longstroke which is a very nice pattern engine but a little pricey. There's also some old OS RF's to be had second hand that you might consider. Super Tigre's X-61 rear exhaust is again available, at least in the UK.
You can also use a side-exhaust engine with a wrap-around or "swan-neck" header that you'll need to have custom made. www.justengines.co.uk makes those.

As others mention there's also 4-strokes, OS 91 with pump, YS 91/1.10 etc that can do the job although at a higher cost than the two-strokes.